I just read some people talking about it.
All agreed that Cain had sex with his sister and then they had many kids cuz they lived so long. The kids had sex with each other and so on there were more people.
The people didnt agree if its allowed by bible, but bible never says "incest is not allowed", bible says that homo-sexuality is not allowed, but incest is not clearly forbidden.
Also bible says that Noah had only few people with him, who are told to be the only people to survive the global flood. This would lead to incest once again.