With whom did Cain and Abel marry? I saw a similar question But the answers wasn't clear to me. Simply how did Cain and Abel have children?
Genesis 5:4 "And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters"

https://www.bible.com/bible/1/gen.5.4.kjv

They had children by their sisters.

(Incest was not prohibited at the time since genetic decay had not set in yet)