Sadly, not every child gets the mom they deserve. Your niece seems to be one of them. She doesn’t have a mom, she has a teenager who doesn’t yet want to be an adult. Your sister seems to be trying to reclaim the teen years she missed. She can’t. But she doesn’t realize it yet. Meanwhile, she has a toddler who needs her to grow up — fast!
The silver lining in this situation is that your niece has other people in her life who are prepared to love her and take care of her. Since everyone has tried to talk with your sister with no results, all you can do is what you are doing — make sure that a little baby gets the care and love she needs. Make sure your sister knows that it’s better for her to leave her daughter with you than to take chances with leaving her by herself.
And no, this isn’t letting your sister off the hook. It’s making sure that a baby doesn’t get hurt by some impulsive mistake her mother makes. Your sister will eventually grow up and realize that she has a lot to apologize for. But you can’t hurry that up by giving her any more talks. Just love that little girl as best you can.
I wish you well.