Well believe it if you want, but this is the logic of it.
Adam & Eva were the creation of God. No mention of anyone else.
They had two sons, Cain and Able.
Who got married.
Who the fuck too, thier sisters or had God been busy on the other side of the hill and did not tell anyone.
The Bible is a collection of Myths and fables, the Jews wrote it while in Babylon as slaves to keep the faith and tribe together. It turned into what it is today by zealots who want to control you.
It's so flawed in science and history to be a joke if it was not a "Holy Book and The Word of God"
Who told you that? The guy burning your neighbor at the stake while he holds a book that says Love Thy Neighbor & Thou Shalt Not Kill.