My brother had issues with addiction, over 30 years of them. It was never his fault, always someone else, the weather, the world.... and he could not blame his home or his parents, just himself for always thinking that there was never a consequence for his actions and there always is! He was bright, good looking, excelled at everything he tried, made friends easily and sadly believed that he could 'do drugs' and he couldn't, they did for him! Alcohol replaced drugs in later years and that just put the lid on his health issues. As a family we tried everything, you name it, therapy, rehabilitation, money, escorting him to help avoid friends, moving, but he increasingly became irrational and that's because liver damage and the toxic effects cause psychosis, alongside the physical harm.
So we focused on his partner, the children, ensured they had the support they needed, spent time with other members of the family, always had an option for support, a model other than his. Eventually, we also called his bluff and told him that he no longer had power over us, so threats, behaviour, calls, kicking down doors no longer worked and cut him off.
Your sister has a choice, as did my brother, but he never chose to make it despite more help than you'd believe was possible and so the choice was for the rest of us to walk away and live our lives.
Please do all you can to ensure your niece is looked after and stop playing the game according to your sister's rules. Make it clear that she has choice and the only person to blame is her. It's tough, but the option is damage to those around her or leave the one to destroy herself or help herself!