I was curious what the Old Testament laws said about incest, which brought me to the same Leviticus chapters that condemn homosexuality. And interestingly enough, they spell out variations of incest in exhaustive detail, and do include "non-blood" cases:
6 No one is to approach any close relative to have sexual relations. I am the LORD.So it seems that some non-blood relations are put in the same category as blood incest. But what's puzzling to me is that even though these cases are enumerated in lawyerly detail typical of the Old Testament, it still leaves a number of questions unanswered. One crucial unanswered detail, in verse 8, for example: are we talking about your father's wife or your father's widow? Having sexual relations with your father's wife while your father is still alive and they are still married would most definitely dishonor your father. It doesn't help here that the phrasing is talking about "having sexual relations", as opposed to marrying and taking as your own wife. If it had said "do not marry your father's wife", it would have been fairly clear that your father was presumed to be out of the picture at the time. But here it's not so clear. And the context is not helpful. In verse 7, having sex with your mother is prohibited, which based on tradition we may presume the prohibition extends even after your father is dead. But down in verse 20, the same language is used to say "do not have sex with your neighbor's wife", which unless it means to prohibit widows from ever remarrying, must mean that they're talking about adultery, that is, the prohibition refers to a living neighbor's wife, not a dead neighbor's wife. Similarly, verse 16 prohibits having sex with your brother's wife, yet if your brother dies childless, you are required (see Deuteronomy 25:5-10) to have sex with his wife, so obviously verse 16 applies only to living brothers. Thus, I don't see how you can tell from this whether step-mothers and non-blood aunts are meant to be off limits even when they are widowed. (Leviticus 20 reiterates most but not all of the chapter 18 prohibitions, along with punishments. The punishments vary, and it uses more varied language -- "has sexual relations with", "lies with", "sleeps with", and "marries". It doesn't give a much clearer picture.)
7 Do not dishonor your father by having sexual relations with your mother. She is your mother; do not have relations with her.
8 Do not have sexual relations with your father's wife; that would dishonor your father.
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.
10 Do not have sexual relations with your son's daughter or your daughter's daughter; that would dishonor you.
11 Do not have sexual relations with the daughter of your father's wife, born to your father; she is your sister.
12 Do not have sexual relations with your father's sister; she is your father's close relative.
13 Do not have sexual relations with your mother's sister, because she is your mother's close relative.
14 Do not dishonor your father's brother by approaching his wife to have sexual relations; she is your aunt.
15 Do not have sexual relations with your daughter-in-law. She is your son's wife; do not have relations with her.
16 Do not have sexual relations with your brother's wife; that would dishonor your brother.
17 Do not have sexual relations with both a woman and her daughter. Do not have sexual relations with either her son's daughter or her daughter's daughter; they are her close relatives. That is wickedness.
18 Do not take your wife's sister as a rival wife and have sexual relations with her while your wife is living.
19 Do not approach a woman to have sexual relations during the uncleanness of her monthly period.
20 Do not have sexual relations with your neighbor's wife and defile yourself with her.
Ironically, we find a fair amount of incest in the early biblical patriarchs. Abraham's wife Sarah was his half-sister (see Gen 20:12), in clear contravention of verse 9 above (although they predated it). Jacob married two sisters who were most definitely rival wives (see Gen 29 vs. verse 18 above), not to mention his first cousins (which many modern states consider incestuous, but was not incest per Leviticus). Judah impregnated his daughter-in-law Tamar (see Gen 38), though he didn't know it was her. Even so, Lev 20:12 seems pretty clear that both of them should have been put to death. When Boaz marries Ruth, he's some kind of close relation of her late husband, as it is a levirate marriage. All of these variations of incestuous marriage were direct ancestors of King David, who along with his neighbor's wife, and King Solomon and one of his numerous wives, were ancestors of Christ. Keep that in mind next time somebody starts advocating a return to traditional marriage along biblical lines.