Update - DH gave SD15 the option to leave
So after some talks DH learned from BM that SD15 acts the same way at her home. Sounds like BM actually comes down on SD15 more than we ever have for it.
convo between DH and SD15:
DH: have you put some thought into what you want
SD15: it's no my choice it's BM's
DH: so if you got to choose you'd pick going to live with BM instead of just trying to be nice to us the 15 minutes a day you see us?
DH: ok *then walked away
DH and BM talked and BM offered to let SD15 stay for a couple months to see how it goes.
So I'm kind of releived that I don't have to deal with it for a while, but still it's bitter sweet for sure, I wish it could be better.
I think this situation will be the same as her with her advanced school. She thinks the grass is greener on the other side, then when she's actually given what she "wants" she'll realize how wrong she was and come crawling back. DH isn't easy to forgive so we'll see what happens. Whatever, good riddance, more time to focus on my littles.