Do we know what was Esther's relationship to Mordecai, based on the Jewish and Christian tradition?
There are two views on the relationship between Mordecai and Esther.
That she was his wife, is shown in one translation of the Septuagint.
That she was his adopted daughter, is shown in the Hebrew (per Josephus) and translated Masoretic texts
Esther 2:7 Septuagint
And he had a foster child, daughter of Aminadab his father's brother, and her name [was] Esther; and when her parents were dead, he brought her up for a wife for himself: and the damsel was beautiful.
Esther 2:7 Masoretic
And he brought up Hadassah, that is, Esther, his uncle's daughter: for she had neither father nor mother, and the maid was fair and beautiful; whom Mordecai, when her father and mother were dead, took for his own daughter.
So, was Esther Mordecai’s wife or daughter? We will see that the “for himself” in the Septuagint, is a misunderstanding. Mordecai adopted Esther for his own daughter and yes to bring her to be a wife.
To return to Esther, the king sent out a command that the best virgins were to come to him (2:2-4). If Mordecai had married Esther, this would exclude her; but, she was included in the search (2:8). This tells us clearly that they were not married, though Esther was of marriageable age. Mordecai was bringing her up to be married, though not to him.
We can also find this same line of thought in Josephus. He was aware of the Hebrew text. The Greek Septuagint appears to be a mistranslation.
Preface to Antiquities
2. Now I have undertaken the present work, as thinking it will appear to all the Greeks worthy of their study; for it will contain all our antiquities, and the constitution of our government, as interpreted out of the Hebrew Scriptures. http://www.biblestudytools.com/history/flavius-josephus/antiquities-jews/preface/chapter-1.html
Antiquities Book 11 Chapter 6 Paragraph 2
So when a great number of these virgins were gathered together, there was found a damsel in Babylon, whose parents were both dead, and she was brought up with her uncle Mordecai, for that was her uncle's name. http://www.biblestudytools.com/history/flavius-josephus/antiquities-jews/book-11/chapter-6.html
So, in the Jewish and Christian tradition, the answer is Esther was Mordecai’s adopted daughter. He was bringing her up in the norms of the day. She was raised a virgin, was never Mordecai’s wife, and became married to the king.
Notes For an outline that the Septuagint was variously translated see https://en.wikipedia.org/wiki/Septuagint
PS I suspect the "married" idea of "old guardian" to "young virgin" Esther 2:7 translation of the Septuagint was made later to conform it to the later tradition from the Infancy Gospel of James of its tale of the "old guardian" and "young virgin".
What was the biological relationship between Esther and Mordechai? Were they cousins or uncle and niece? And was Mordechai Esther’s adoptive father or even her husband?
The Hebrew Masoretic text is straightforward, on Esther 2:7,
He was foster father to Hadassah—that is, Esther—his uncle’s daughter, for she had neither father nor mother. The maiden was shapely and beautiful; and when her father and mother died, Mordechai adopted her as his own daughter.
The Hebrew Masoretic Esther 2:7.
According to the Hebrew Masoretic text, Esther is the daughter of Mordechai’s uncle, and thus, Esther and Mordechai are first cousins. When she was orphaned, Mordechai adopted her. Naively, that should close the matter, but as almost anyone who has studied this debate knows, it is not that simple.
Tradition that favors Queen Esther as Mordecai's wife
According to the Rabbinical Jewish tradition, Queen Esther was betrothed and wedded to Mordecai her first cousin. In the Eastern Christianity: 23 sui juris Eastern Catholic churches, 16 canonical Eastern Orthodox churches, and 6 communion Oriental Orthodox churches agree with the Rabbinical Jewish tradition. This tradition can be traced from the Old Koine Greek text of the Old Testament for the Divine Liturgy. In the Greek text of the Book of Esther, the passage describing the relationship as such:
Now there was a Jewish man in the city of Susa whose name was Mordecai, son of Jair, son of Shimei, son of Kish, of the tribe of Benjamin, who was an exile from Jerusalem, whom Nebuchadnezzar the king of Babylon had taken captive. He had raised the daughter of Abihail his father’s brother, and her name was Esther; and when her parents died, he brought her up to be his wife, and she was beautiful.1
The Greek Septuagint Esther 2:5-7.
Tradition that favors Queen Esther as Mordecai's adopted daughter
Some may raise an objection to this tradition by contending that if Esther was Mordecai's wife, that raises a lot of questions and yet in the Sacred Scriptures we cannot find anything except that she was raised by him as the guardian and that she was adopted as his own daughter! A classic example comes from Josephus:
So when a great number of these virgins were gathered together, there was found a damsel in Babylon, whose parents were both dead, and she was brought up with her uncle Mordecai, for that was her uncle's name.
How to reconcile the two apparent contradictory traditions?
First we need to note that the text from Josephus taken by itself doesn't contradict the Greek text that Esther was betrothed and wedded to Mordecai her first cousin. Esther was adopted as his own daughter and Mordecai married his first cousin.
Second Josephus was mistaken in calling Mordecai as Esther's uncle, because Mordecai's father and Esther's father were brothers (rf. Esther 2:7,15).2 The correct relationship between the two is not uncle and niece but rather as first cousin.3
He was bringing up Hadassah, that is Esther, his uncle’s daughter, for she had no father or mother. Now the young lady was beautiful of form and face, and when her father and her mother died, Mordecai took her as his own daughter.
Now when the turn of Esther, the daughter of Abihail the uncle of Mordecai who had taken her as his daughter, came to go in to the king, she did not request anything except what Hegai, the king’s eunuch who was in charge of the women, advised. And Esther found favor in the eyes of all who saw her.
From five texts of the Book of Esther we can conclude that Esther was the daughter of Mordechai’s uncle, i.e., his cousin, with only one anomaly, like Josephus, makes her his niece.
The relationship of Esther and Mordecai as cousins is evidenced by the identification of Esther in 2:7 as the daughter of Mordecai's uncle and as the daughter of the brother of Mordecai's father. "According to the Semitic custom, the most suitable wife was a cousin on the father's side, which was the relation Esther was to Mordecai."4
This support the tradition that Esther and Mordecai were married when Esther was taken by force to the king. This is why in Esther 2:10 we find that she was charged by Mordecai to remain silent. If supposed she told the king of her lowly origin there shouldn't be any difficulty, because the king guards who forcibly kidnap her knew of her origin. The only crucial information that she kept from the king is her betrothal status to Mordecai. Mordecai in the Greek text is described as a prophet of God who received revelation of the coming plot of assassination. In Esther 2:11 we find how Mordecai as a faithful husband walked in front of the court of the harem daily to keep in contact with Esther his faithful and lawful wife, who was forced into unlawful marriage to the king.
Esther had not made known her people or kindred, for Mordecai had charged her not to make it known. And every day Mordecai walked in front of the court of the harem, to learn how Esther was and how she fared.
RSVCE Esther 2:10-11.
Addressing the Questions raised on the Tradition
The Hebrew Masoretic text Book of Esther 2:7 says, "And he [Mordecai] adopted Haddasah [Esther] ... and when her mother and father died, Mordecai took her to him as a daughter."
There are three apparent anomalies in this verse. First, since the verse says that Mordechai "adopted Haddasah," why does it seem to repeat the fact that he "took her to him as a daughter?" Isn't that the same thing? Second, there is no legal status of "adoptive parent" in Judaism; that is, you raise an orphan girl in your home, but you don't "take her as a daughter." Finally and most notably, "took her to him" is always used in the Torah to refer to marriage. Literally, then, the verse is saying that he married her.
Why does it use the term "daughter" someone might asked? The terms "sister" and "daughter" are common expressions of endearment, as we see in other places in the Old Testament (e.g., Ruth 2:8-95, 2 Samuel 12:36, Song of Solomon 4:9-107). The idea is that a husband and wife should develop a loving and giving relationship as one naturally has with one's child and sibling. So, it's not hard to see how the Greek text in this verse support for the tradition that says Mordechai, Esther's cousin, was also her husband. In fact, the ancient Near Eastern practice of adoption by marriage, "in which a man would adopt a child with the intention of marrying her when she was old enough,"8 support this tradition.
"[The king] wished to taste the taste of a virgin, and he tasted; the taste of a non-virgin, and he tasted."9 According to the Rabbinical Jewish tradition, this view is based from the usage of both 'women' and 'virgin' referred to in Esther 2:17. This tragedy is not unique, in history we had Caligula a Roman Emperor who similarly took both virgins and married women at will.
The king loved Esther more than all the women, and she found favor and kindness with him more than all the virgins, so that he set the royal crown on her head and made her queen instead of Vashti.
According to the Jewish laws, "she was engaged in an involuntary sexual relationship with the king, or, in other words, she was being raped regularly by him. [...] what went on between Esther and Ahasuerus was not adultery because it was non-consensual."10 This explains Esther's initial passivity in her relationship to the king. Esther was in an involuntary, forced relationship. She was the object of the action being taken and her induction into the harem was beyond her and Mordecai's ability to deflect. Esther's compliance with the sacrifice in her life "reflects an adaptation strategy fitted to the realities of her situatedness."11 A strategy that enabled her to survive and ultimately succeeded in assuming a role that saved her people. Her life is a typology of Christ's sacrifice that saved His people.
The Consensus of Scholars
The Fathers of the Church considered Esther as a type of the Blessed Virgin Mary and Mordecai as a type of St. Joseph her guardian and husband.
Long before Luther, according to Jaroslav Pelikan, ancient Jewish authorities objected to the book’s canonicity. But early Christians loved it. In his book The Rest of the Bible Theron Mathis mentions several church fathers who referenced Esther approvingly: Clement of Rome, Athanasius of Alexandria, Ambrose of Milan, and Aphrahat the Persian. Even Jerome, who discounted portions of the book, saw the book’s principal characters, Esther and Mordecai, as types of the church and Christ.
Were these fathers reading the same book as Luther? Actually, no.
As far as I know, there is only one author Hanna Kahana who assert that the Greek Esther 2:7 is a corrupt translation from the original text by assuming that the Hebrew Masoretic Esther 2:7 is more likely to be an accurate of the Old Hebrew text.
It may be considered as an exegetical translation, in which case it is a very controversial one [...] the Translator read בית and not בת, and interpreted this as took her as a home and hence as a woman, in the sense of spouse, but this explanation seems to me not very plausible.
Hanna Kahana, Esther: Juxtaposition of the Septuagint translation with the Hebrew text.12
As we can find evident among the scholars such as: (1) Michael V. Fox, (2) Eliezer Segal, (3) Barry D. Walfish, (4) Joshua Berman, (5) D. J. A. Clines, (6) Lewis B. Paton, (7) Kristin De Troyer, and (8) Karen H. Jobes which have been cited to support the Jewish and Christian consensus. Kahana's assertion that the Greek text is corrupt can't be sustained in light of consensus among scholars. That the Jewish and Christian tradition on Esther as Mordechai's cousin whom he raised as a daughter and later to become his wife. Which is common in Jewish culture at the time of Babylonian exile.
After such a meticulous comparison of texts, Kahana concludes 'most differences between Μ and S must have been due to translational decisions of the Translator himself, rather than to a different Hebrew Vorlage or to subsequent scribal errors or other corruptions in transmission' (p. 442, emphasis mine).13 Her conclusion lacks an awareness of the rather complicated textual history of the transmission and redaction of the texts of Esther. She seems unaware that her conclusion is contrary to that of both Clines (D. J. A. Clines, The Esther Scroll: The Story of the Story (Sheffield: JSOT Press, 1984)) and Fox (Michael V. Fox, The Redaction of the Books of Esther (Atlanta: Scholars Press, 1991), fails to cite scholars whose work might support her conclusion, and does not adequately present a summary of evidence to support her conclusion. [...] Perhaps the greatest weakness of the book is its lack of substantial engagement with the work of others on the Greek texts of Esther, especially work similar to Kahana's previously done by De Troyer (Kristin De Troyer, The End of the Alpha Text of Esther. Translation and Narrative Technique in MT 8:1-17, LXX 8:1—17, and AT 7:14-41 (SCS 48; Atlanta: Society of Biblical Literature, 2000) and by Jobes (Karen H. Jobes, The Alpha Text of Esther. Its Character and Relationship to the Masoretic Text (SBLDS 153; Atlanta: Scholars Press, 1996), both of which contain material relevant to the ό-text. References to the work of others are found in footnotes that typically indicate only whether Kahana agrees or disagrees with them, but there is no substantial engagement.
Karen H. Jobes,14 The Journal of Theological Studies, New Series, Vol. 58, No. 2 (Oxford: Oxford University Press, October 2007), 590-2.
Lewis B. Paton exegetes the Greek Esther 2:7 by concluding that the translator copied from a different Hebrew text than from what Masoretic copied.15 This is the dominant view among scholars, with the consensus agree that the Greek Esther 2:7 is more reliable text and consistent with the Jewish and Christian tradition.
[Some] were troubled to see how Mordecai could take a girl of his own generation into his house as a daughter. According to Semitic custom, a cousin on the father's side is the most suitable of all persons to take as wife (cf. Ar[amaic] bint 'amm, 'daughter of paternal uncle,' as a synonym for 'wife'). Meg. 13a solves the difficulty by reading lebheth, 'for a wife,' instead of lebhath, 'for a daughter' (in Heb[rew] beth, 'house,' has the secondary meaning of 'wife'), and justifies this interpretation by 2 S[amuel] 12:3, where 'like a daughter' is parallel to 'slept in his bosom.' This view has been followed by G[reek text], and has found wide acceptance in the Targums, Midrashes, and comm[entaries]. It must be admitted that nowhere else is a wife of Mordecai mentioned [other than in 2:7].
Lewis B. Paton, A Critical and Exegetical Commentary on the Book of Esther.
1 Footnote on the Book of Esther 2:7 in the Orthodox Study Bible (2008) explains, "Mordecai's relationship to Esther is that of both guardian and husband, a necessary social and legal arrangement and not unusual at the time."
2 LXX Esther 2:7, "He had raised the daughter of Abihail his father’s brother."
3 The tradition differ on the matter of Esther's age but it agrees that they were husband and wife not uncle and niece as mistakenly popularized by a 2006 rendition of the Book of Esther in One Night with the King.
4 Tricia Miller, "Jews and Anti-Judaism in Esther and the Church" (Cambridge: James Clarke & Co, April 2015), 132.
5 NASB Ruth 2:8-9, 'Then Boaz said to Ruth, "Listen carefully, my daughter. Do not go to glean in another field; furthermore, do not go on from this one, but stay here with my maids. Let your eyes be on the field which they reap, and go after them. Indeed, I have commanded the servants not to touch you. When you are thirsty, go to the water jars and drink from what the servants draw."'
6 NASB 2 Samuel 12:3, "But the poor man had nothing except one little ewe lamb which he bought and nourished; and it grew up together with him and his children. It would eat of his bread and drink of his cup and lie in his bosom, and was like a daughter to him."
7 RSVCE Song of Solomon 4:9-10, "You have ravished my heart, my sister, my bride, you have ravished my heart with a glance of your eyes, with one jewel of your necklace. How sweet is your love, my sister, my bride! how much better is your love than wine, and the fragrance of your oils than any spice!"
8 Michael V. Fox, Character and Ideology in the Book of Esther (1991), 276.
9 Eliezer Segal, The Babylonian Esther Midrash: A Critical Commentary, Volume 2 (1994), 65.
10 Barry D. Walfish, Kosher Adultery? The Mordecai-Esther-Ahasuerus Triangle in Midrash and Talmud (2002), 118-9.
11 Joshua Berman, Hadassah bat Abihail: The Evolution from Object to Subject in the. Character of Esther (2001), 649.
12 Hanna Kahana, "Esther: Juxtaposition of the Septuagint translation with the Hebrew text", Contributions to Biblical Exegesis and Theology 40 (Lueven: Peeters Publishers, 2005),84-5.
13 Emphasis in the citation is original from the author and not mine.
14 Karen H. Jobes (PhD, Westminister Theological Seminary) is the Gerald F. Hawthorne Professor of New Testament Greek and Exegesis at Wheaton College and Graduate school in Wheaton, Illinois. Her 1995 PhD dissertation from WTS is "The Alpha-Text of Esther: Its Character & Relationship to the Masoretic Text", published by Society of Biblical Literature Dissertation Series 153 (Scholars Press, 1996). She is the English translator of Codex Sinaiticus Esther for British Library which is included in "New English Translation of the Septuagint: Esther" (Oxford: Oxford University Press, 2007).
15 Lewis Bayles Paton, "A Critical and Exegetical Commentary on the Book of Esther" (New York: Charles Scribner's, 1908), 171.
Esther 2:7 VUL
And he had brought up his brother's daughter Edissa, who by another name was called Esther: now she had lost both her parents: and was exceeding fair and beautiful. And her father and mother being dead, Mardochai adopted her for his daughter.
And as the time came orderly about, the day was at hand, when Esther, the daughter of Abihail the brother of Mardochai, whom he had adopted for his daughter, was to go in to the king.
"Brother" can refer to a 'cousin' or 'close relative' among the Jews.1 So Esther is either his niece or more loosely 'female relative' of whatever kind that may be.
1 e.g. Gen. 14:16; 29:12, 15