The best thing you can do in your situation is pass this site along to your brother-in-law. I know that he is trying desperately to help his wife/your sister but really all of it to me seems like enabling. As long as she has a roof over her head and someone to pick up the pieces of her destruction she will continue on this path.
Its hard to hear your story because I know you feel very helpless. Because of the fact that you mentioned she has gotten violent with her child the best thing to do (IMHO) is for her husband to ask her to leave.
Its no longer about your sister its about her child. Things will continue to get worse. No good can come of her staying in the home.
I dont know the legalities or the ins and outs of laws but what I do know is that you BIL is living the life I chose not to. I asked my exabf to leave our home over a month ago. Today he is in treatment. I am unsure of what will happen to our family but I know that I could not continue allowing an active addict to live in my home with our children.
That was one of the most painful things I have ever done. But my kids suffered alot from it. My oldest child suffered the most and in ways my ex will never understand until he reaches that point in his recovery, which could really be never. The final straw for her was when he came and removed his things from our home. It was like she snapped. I had to have her hospitalized. She is home and better today but the damage is done. I am the parent with the clear head and it is my job to protect my children at all costs. There is no greater love for your kids then to show them that they mean more to you then someone who is destorying themselves and everyone in their paths.
What will happen to your niece if she continues to witness the downward spiral of her mothers addiction. Ask your BIL to come to this site and read and educate himself to the life he is choosing for his child.
Keep posting. Keep reading...Thoughts and prayers are with you and your family...