Yahweh God is a moral God. He can not change His morals or what is right or wrong. I like to tell people that what was sin the day The Adam was placed in the garden is still sin today. What is sin today was sin that very same day. God can not change. To change what is right and wrong from one day to the next or even from one covenant to the next changes God into another God. Therefore incest as defined by scripture has always been a sin. So who did Seth marry?
Scripture does not say, but it does give us clues. Read Genesis 1 and 2. I believe that Genesis 1 is the general creation. This general creation included Adamites. Let me suggest that God created several Adamite couples. Genesis 2 is the specific creation of, as the Hebrew text says, “The Adam” in the garden.
Genesis 1 children were then available for Seth, and Cain as well, to marry. There may be other solutions. Study and consider the possibilities.