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...