A tough one, TAB.
Even though your Mom is trying to keep her grand kids in the family, it isn't a workable long term, or maybe even short term, solution.
You know that or you would not be thinking of going against your Mom's efforts.
It is what is best for the kids and what should be done.
You will have to pay the price, as Tom B. has pointed out, with your Mom and any supportive family members. Tough it out. Better to face that than have to live with letting the girls lives be hurt.
Do what you can to keep track of them and what you can to watch over them and help them. Make the foster family part of your family and do the same with the adoptive family, if possible. Hopefully the rest of your family will respond the same way.
Good Luck, TAB, and take care.