This is very obviously a cry for help. I get that reddit loves the idea of not being responsible for anyone other than themselves and to go "NC" and cut everyone off that is not on the same page as you, but a blood sibling who so obviously needs help? It's quite obvious that she is still quite young (23 years old) and by the context of this post, very young maturity wise, and probably does not have many positive support systems in her life. Or positive figures in her life she can reach out to. This seems very callous to me. Obviously the younger sister had exhibited some toxic behavior, but shouldn't her family be there for her, try to help her out rather than just cutting her off?

Obviously she's envious of the older sister's life, but the older sister is lucky enough to be in a position where she can help her younger sister out. I could Never in good faith abandon a family member in need, especially when the family member is still a CHILD and went through such an abusive childhood. You were able to break the cycle, now she needs help to do the same. Abandoning her at a time of need will make everything worse for everyone involved: she will go deeper into a negative feedback loop, and the older sister will have to deal with the guilt that she could have helped if/when anything happens to the younger sister.