But there's a lot to do to learn more about this dynamic and ID wherein it might have originated in her--and help you and other family members learn about relational dynamics. Lots of good stuff to learn about what we do right . . . that's why I can't recommend Bancroft's work enough. He's worked with batterer groups for 20 years--and he links their strategies to larger politically oppressive strategies too.
He was the first to make the clear argument that their behavior originates quite logically out of their (distorted) beliefs, not their "emotions." He applied cognitive psychology to what they're doing. It's really eye-opening and helps you at once dispense with bullshit guidance from the many misguided therapists and others who don't know any better.
There's a lot of insight that can help you and your family personally and politically while you work to let go of your illusion that you all are responsible for your niece's choices.
I know it's hard. It's like overcoming addiction--a minute at a time, an hour at a time. And expect to backslide. But there's a LOT you CAN do to further understanding of relational abuse even if you can't influence your niece, especially given that you're a male and young men can benefit from your example. We need you!