In the story of Cain and Abel that comes from the first book in the bible, the book of Genesis, from the jewish written Torah (the first 5 books in the bible, aka the books of Moses), Cain kills his brother Abel out of jealousy. Then God casts him out to wonder the world, The bible says Cain took his 'family' and went. Before Cain and Abel, their mother and father were Adam and Eve, the first humans created by God. How is Cain married with children if they were practically the only humans at that time? Because in the Talmud of the Jews, we are told the only humans are jews and that the rest of humanity do not come from God but are made to look like humans, so that the Jews are not served by beats of the wilderness.

Go ahead and research this yourself to start to understand that what you think about what is going on in the bible is not correct.