Sounds like you are trying to keep the family together. But I also think its important not to immediately label your niece as a “toxic” person, try to recall what your world was like as a 14 year old. The smallest thing was practically the end of your life and getting advice from adults was the last thing you want.

Instead, maybe consider her a troubled teenager and look up articles regarding dealing with rebellious behavior and how to handle that. Here is a place to start: http://www.lifescript.com/well-being/articles/h/how_to_deal_with_a_rebellious_teen.aspx