Hi T.,
It sounds like she might have some issues over not having her mother in the picture, she sees you with yr children and is probably lashing out if her mother is not in the picture, I gather that if u watch her on the weekends that her mother isn't around. Tell yr brother he needs to get her help now, before things even get worst. She might need someone other then family that she can talk to on how she feels, she sounds like she has alot of anger, and is to young on how to vent that anger and not on family that is there for her.good luck let us know how things are going.