I ask because I'm trying to understand whether the prohibition against sibling marriage is due to genetic or emotional reasons.

Suppose Bob and Alice marry, have a daughter Eve, and some years later Alice dies.

Suppose Chris and Diana marry, have a son Frank, and some years later Chris dies.

Now, suppose Bob and Diana marry -- that would make Eve and Frank stepsiblings with no shared biological heritage.

Would the Old Testament law, in this case, prohibit Eve and Frank's marriage?


2 Answers 2


The prohibited forms of incest are found in Leviticus 18.

Verse 9 is of the most concern:

9 “‘Do not have sexual relations with your sister, either your father’s daughter or your mother’s daughter, whether she was born in the same home or elsewhere.

Verse 11, however is also interesting:

11 “‘Do not have sexual relations with the daughter of your father’s wife, born to your father; she is your sister.

Here, only children born of the father are considered. As such, a step sibling sired by another man (such as the case you posit) would appear to be fair game.

The complete list disallows:

  • your mother
  • any woman your father marries (honor)
  • your sister, or the daughters of either of your parents
  • your children's daughters (honor)
  • your father's wife
  • your aunt on either side (she is "a close relative")
  • your daughter-in-law
  • your sister-in-law

In many cases, the point seems to be "don't dishonor your family." I suspect (and here I put on my Rabbi hat and accent), the idea would be that you could not seek out your step-siblings, but if the marriage were to occur before your father and her mother were to marry, that they would be dishonoring you, not you them.

All of this, of course, is far more legal and lawerly than Christianity normally gets, but you asks the question, you gets the answer...


The Bible doesn't specifically address this. Unfortunately, The Bible isn't clear on the reason for not marrying your sister, and the idea of non-blood relations isn't addressed. There's actually a chart on Wikipedia matching verses to types if potential incest, and for step siblings, there's nothing.

This is not to say it's okay or not. There are plenty of specifics the Bible does not address specifically.

  • It does not specifically mention masturbation.
  • It does not mention whether or not it's okay to buy your wife flowers.

When there is no clear Biblical statement in a moral issue, such things are next determined based on extrapolating Biblical principles. But the answer to your question remains "there's nothing specifically forbidding it, and genetic similarity isn't cited as the reason for the prohibition on sexual relationships with close relatives."

I still don't know if that makes it okay, though. A better follow-up question that may have more definitive answers, could ask for the Catholic view on this, or another denomination.


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