I believe Adam & Eve had more kids than just Cain & Abel, then their kids had kids and so on. People can multiply pretty quickly. In about 13-15years the next generation was born and so on and each person could have like 6-10 kids. So if Adam & Eve had 10 or more kids and each of those kids had 6-10 kids 13-15yrs later, people can multiply pretty quickly by the time Cain was like 30yrs old so it could have been any of his nieces, or cousins. There was no such thing as waiting till you’re 18yrs old to get married back then.

I know someone who had 6 kids and 30 grandkids and imagine those 30 grandkids each having 6-10 kids of their own. And that’s today with people waiting till the age of 18. So imagine people starting at 14yrs old. So they multiplied pretty quickly back then then split off into their own little groups.