Thank you, Ellie, for an excellent question! I can tell you are a good thinker and am glad you are looking for an answer.
Things so long ago were different from our lives now. Adam and Eve and their children were the only people in the world. God gave Adam and Eve MANY children. Cain was the oldest. Abel was his brother. When they were grown up they fought, and Cain killed his brother because he was jealous. God sent Cain away and gave Adam and Eve a son named Seth. Seth’s family, a long time later, were Jesus’ kinfolks. Adam and Eve had many other children besides those three. In fact, Adam lived over 900 years and had a LOT of kids and grandkids. Some of Jesus’ relatives thought Adam and Even had over 50 kids!
When Cain moved away, he took one of his sisters or nieces as his wife but God didn’t tell us her name. They had many children. Enoch was the first son they had. It wasn’t against the rules to marry your cousin or sister until over 1,000 years later. That’s when God gave Moses the Ten Commandments and the Law. He told Moses he didn’t want people to do that anymore. By then, there were plenty of people to choose from.
Before God told Moses the new rules, Abraham married his half-sister. Isaac and Jacob both married their cousins. God’s people even had more than one wife sometimes, but God told Moses a long time later that He wanted people to get married to God-loving people, and stay married to each other all their lives.
That’s still what God wants. But many people want to do the things God doesn’t want them to do because they don’t like God to be their leader. God wants us to be wise and to live well because He wants the very best for us. He loves us more than anything! We have to decide who we’re going to let lead our lives. I’ve learned that when I do things God’s way, everything feels better.
Thank you again for your question, Ellie. A lot of grownups wonder about Cain, too, but they’re not brave enough to ask their question the way you have.
I hope you grow up and marry a God-loving man and have a good and long life with a great family.
Passages to consider: I Cor. 15 (esp. v. 45); Gen 3:20; Gen 4:17; Gen 5:4; Leviticus 18
Here’s a video with more information: https://www.youtube.com/watch?v=qZojhvVgGhQ