evalycra, they haven't stopped it because they don't want to.
I don't know what the family dynamic is, but often there is a weaker child who the parents continue support because if they don't, they have awful fears about what will happen - the child will end up in prison, on the streets, dead, etc.
The other children are capable of taking care of themselves, in the parent's estimation, but the one they continue to take care of into adulthood isn't.
If your parents in law are of sound mind and capable of managing their own finances, I think I'd leave it alone - especially since you aren't their child, you married into this family.
I would take her aside if u have other siblings get them involved as well.someone needs to say something