Oh Kat, what a painful mess. I am so sorry for your sister. I really don't have any advice. This is so beyond me. I just know if it were my baby sister, I would be doing everything I could to get him out of her life. There just comes a time when you don't have any control when they reach certain age.
One thought I have is "getting custody of your niece and holding that over her head that as long as she is involved with that man that you will fight her for custody of her daughter" That worked for some friends of ours and their daughter is a completely different person and has since married a great guy and is making a good home for her daughter. Just sharing that one for what it is worth.
Just know that I am thinking of you and will be praying for you.
Love you dear friend.
Possi
