My question to everyone - - - how can a BPD know they need to get help if nobody tells them? I mean, I don’t mind being the bad guy and telling my BPD sister she needs help with her rages and alcoholism if that would help her in the long run. Has anyone tried doing this? Do you have advice? (But on the other-hand I would be just fine with NC forever too.)
You know the phrase, "You can lead a horse to water, but you can't make him drink"?

I can understand why you want to tell your sister that she needs help. At the same time, she needs to be ready to accept that and work on herself. How do you think she'd react if you told her that she needs help?
It sounds like your niece is going through a tough time also, and you're involved with both of them. How are you holding up?