She has a partner and a kid of her own; she can’t be responsible for all of this craziness.

It’s easy to feel pressured to help in a dysfunctional situation where there are kids involved. You probably wouldn’t house the most irresponsible members of your family if they were bachelors and bachelorettes. Still, if they had a couple of ragamuffins with nowhere else to go, you might feel more inclined to help them out. 

Manipulative people know that normal people have a soft spot for kids, so they will use their kids to get their way. They’ll leave their kids with family members without prior warning to go off and do God knows what, because they know their family members are better people than they are and will take care of those kids because they have a moral duty to do so. You’d be surprised how many people get through parenthood by just offloading their kids to the nearest adult until that adult gets fed up dealing with them, then quickly moves on to the next adult who will tolerate their baloney. It takes longer than you’d think for certain adults to get fed up with their family members’ consistently irresponsible behavior.