But if incest isn't defined by the Mosaic law, then how is it defined? If it's the application, not the moral principle, then what is the moral principle that applies?
Incest is one of the sins drawn from the broad application of the seventh commandment, hence related to the sins of fornication and adultery, cf. Westminster Larger Catechism #139 (above).
The "Mosaic" Law, e.g. Leviticus 18, was part of civil law given Israel for Israel, a particularized form of law given for the unique constitution of the Old Testament theocracy of Israel, not strictly applicable to all other nations.
Here is a summary of Leviticus 18 prohibitions under this category:
The following regulations can be gleaned from this section. A man could not marry his mother (Lev. 18:7) or his sister (or half-sister; first degree of consanguinity according to modern genetics, v. 9). Prohibition of marriage to one’s daughter was assumed and so not mentioned (cf. Gen. 19:30-38). Nor could he marry his granddaughter (second degree of consanguinity, Lev. 18:10) or his aunt (father’s or mother’s sister, vv. 12-14). Also forbidden were marriages to a widowed wife of a close blood relative, that is, a man’s stepmother (v. 8), half-sister (v. 11), aunt by marriage (v. 14), daughter-in-law (v. 15), sister-in-law (v. 16), step-daughter (v. 17), and step-granddaughter (v. 17). The apparent reason behind these prohibitions against marriage to those with such an in-law relationship was that they had become “one flesh” (Gen. 2:24) with the blood relative, and so were regarded as a part of the larger family unit (cf. Wenham, Leviticus, pp. 254-8, for specific examples of some of the more difficult identifications).
Do you see that the "Mosaic Law" included even some relations that were not by blood, but rather by law?
There were reasons based on notions of extended family, and preserving tribes that applied, and thus were part of the civil law given Israel.
That doesn't make the law without moral principle, but it does show how the particularized law given Israel, was civil law given only them.
Skyler, you still have not defined what exactly you are referring to when you refer to "incest"- is incest within four degrees of consanguinity? three? two? Does it involve in-laws? step-parents?
We don't have this issue with adultery- it is sexual relations outside of marriage. That's commandment Seven, and it is moral perpetual law.
And of course there, the Word of God gives us a basis for understanding the broad application of commandment Seven, including our Lord's teaching about it in the Sermon on the Mount.
One question you might be asking is whether the full import of Leviticus 18 was binding beforehand, e.g. on Adam and Eve.
I don't think so.
It seems implicit that close intermarriage was necessary at first, certainly in violation of Leviticus 18.
Perhaps, along the way the gene pool got progressively mutated to the point that at the time of Leviticus, God gave a particularized form of law to Israel.
Now that is reasoning that is speculative, the reason is not given in Scripture.
But it seems clear that a Leviticus 18 definition of "incest" was not binding ab initio, nor is it strictly binding on all nations today.