It is an unfortunate situation for your niece. Your sister is selfish and self centered to think of no one. Especially, her daughter and put herself above all else. She has an innocent life depending on her for guidiance and most of all love. I'm not saying your sister hates her daughter but if she spends her time with all those different men and partying where does that leave her daughter? Shove in the corner and mommy will get to you whem mommy feels like it...that child should be the most important thing in her life. Her well being and safety is pretty much in her mother's hands. And if she can't take on that responsibility then she needs for someone to step in and do it for her.
Can your family babysit her daughter? I know it's an easy way for her but the benefit does go to your niece at least she is safe with you guys rather with your sister with all the many men going in and out of the apartment.
Too bad your sister is old enough to really don't give a care if she doesn't want to. No matter what you say or do, if she's not going to change no one will make her. And she may not wake up until something drastic happens...let's just hope that it doesn't go that far.
Maybe after the divorce she is seeking for attention....or maye it's a cry for help. There are people that does crazy stuff but really deep down inside they are suffering. They do not know how to verbalize rather it's through their actions alone.
Best of luck...here's hoping your sister grows up and be a good role model for her daughter...she deserves that much.