masterjedi wrote:no. family members share some of the same genes so after the third child the gene mixture is upset because the same genes will result in the child becoming deformed.

It took me five times to read and understand what you meant, take some time to structure your sentences more thoroughly.
Incest/inbreeding leads to a less diverse gene pool within the family/group which can cause some genetic defects and weakening the resistance to diseases of the individual. It is not set in stone that a child/offspring will inherit genetic defects or deformities, the chances of it happening with incest is just much higher than with interbreeding.
In my view: If family members wish to marry each other, they can do so. Though, if they wish to commit to a child, they should be aware of the potential risks with inbreeding... or they can adopt a child and help foster the child's development.
Cheers