The truth is there is very little you can do, but you CAN be there still for your sister. Letting her know there is someone willing to listen to her and not judge her is more important than anything. there will come a time when she needs someone to talk to about whatever is bothering her and if she has you to chat with that alone maybe enough until she's old enough to leave. If you have the ability to, when your sister is old enough (18) let her know she can either stay with you for a time or that you would enjoy having her move closer to you, maybe enough to let her know she is loved. For now showing her what a healthy two way relationship is all you can do. It's still a huge undertaking but It's also a really valuable thing to teach her. Is she on social media or does she have a phone of her own? I didn't at that age but the worlds changing. If not you may have to put up with your Nmom in order to talk to her, Something you should think about and be prepared for, even if she has her own phone/media account.