OK I don't know how good of advice this will be since my parents are still married and dh and I are still married. But dh had divorced parents and he had issues up until he was 25 or 26 about his mom and dad.
His dad was the absent parent and dh had a lot of anger about it his little sister wouldn't listen to one bad thing said about him. He treated them the same said he would come and didn't, said he paid child support never did (this was before they could take it out of paychecks but mom got the house because of it), said he never cheated OMG that was a whopper one!!
Anyway now dh has a relationship with his dad dh just accepted his dad would always be irresponsible and selfish but in his own way loves us. This lets dh let go of the anger and enjoy a relationship (a weird one but it works) little sister doesn't want to have anything to do with him.
So you really can't explain parents and their realtionships with their kids.
If you really feel she is in harms way I would go to court.
If not I would say if mommy doesn't like what we buy you how do you feel about leaving it to use when you are here? The mommy works hard well I would say all mommys and daddys work hard because we love you!
I am sure it is really hard to hear things repeated.
I also know that my girls sometimes will say things that they know will make me feel bad and do or give in to something I have already told them NO Way. I'm not saying this is happening but all kids learn this and if she is your oldest you might not be aware of it yet. Again I'm not saying that she is or may ever but my teenagers do this and Sometimes I realize it right away and call them on it and sometimes it takes me a while

__________________
Karen