There are a lot of cases in past history regarding Ruling Families wherein fathers married daughters and brothers married sisters. Something to do with keeping the bloodline pure, and no one seems to have had a problem with this at the time.
In Hawaii once upon a time this was common practice. And they just bumped off any disabled or damaged children. No one seems to have made a fuss about this either. A bad one. Get rid of it. Have another one. Very sensible under the circumstances. Until The Missionaries came along.
But in Egypt it seems to have been more about the Ruling Family and retaining power. Tutankhamun was disabled, although not massively, although I don't know much about his head, or brain power as such. However, he did die young.
But none of these people seem to have had any trouble mating with Siblings or Parents.
It is now very much against The Law, for obvious reasons. Although I don't think anyone actually does anything about these people living together so long as they don't breed. And who would need or be able to know anyway.