I have been taking care of my niece for 3months because her parents were incarcerated?

They left her with her other aunt that lived in nasty conditions so she came to live with me. Know her parents are out of prison and it's like she is a totally different kid she acts up and throws fits for no reason only to her parents not anybody else. Before the parents came to stay with us she seemed much happier she never did any of the things she does to her parents. She listens and respects my husband and I, she does everything we ask her too with no problem but if my sister even says anything to her she cries. She is only 20months so I'm sure she is pretty confused and probably getting back at her parents for putting her through all the crap and I don't blame her. If anybody has advice we would appreciate it. I would like to see her smile around her parents you know?