How are we supposed to know that, exactly?

Just speculating, I would guess that if mama is a mess and the sister is defending the mama, the sister is probably a lot like mama.

Letting the sister come around probably invites all sorts of drama that nobody really needs.

I am guessing the sister probably doesn't have a lot of positive things to contribute to the development of a child if she is like her mom. I wouldn't want my children getting to attached to relatives that could ultimately bring the child a lot of grief. I have a brother-in-law we didn't want around our child, and as things turned out, we were smart to have made that decision. He is in the penitentiary today.