What is the true issue here? Is it that there is an inconsistency in the Scriptures, or is it that there is a misunderstanding of the nature of revelation on the part of the one who “found” the “inconsistency?”
Honestly, there is a mistake on the part of the one who thinks there is an inconsistency.
Morality takes different forms. For example, homosexuality is immoral because it confuses the diversity that God intended to exist within the marriage relationship. Fornication (pre-marital, or extra-marital sex) is wrong because it neglects that covenant commitment that God has designed to go along with marriage. Incest differs from both.
Why does incest differ? Incest differs because it does not violate any of God’s natural design for marriage. Incest is prohibited later in Scripture. Why? It is prohibited by more recent revelation than creative revelation because of the fact of the fall and genetic issues. Abraham lived close to the beginning of time. Genetic problems as a result of the fall had not multiplied so much as they did later. Thus it was that God waited to reveal His prohibition of incest at a later time, and could do so without being inconsistent.
One thing is definite: the prohibition of incest is like all other moral prohibitions; it is for our good.