That isn't really how that works at all. If you have a brother and a sister who both have male and female children, that's basically the same genetic line going

That isn't really how that works at all. If you have a brother and a sister who both have male and female children, that's basically the same genetic line going on. The only exception I can think of is if you have some mutation they didn't. But in that case if you're aware of the mutation the consequences are likely to be awful, which is a great case for not having biological children.