Hi @Blacksheep, thank you for posting and welcome to the forums 🌻
I'm sorry that you're experiencing this situation - I can relate to complicated dynamics like this. It sounds difficult because perhaps your sister didn't behave in such ways before she became involved with this person? So the behaviours could just be due to the unhealthy influence of that person, and if so it might not be easy for those behaviours to change if they're together often. At the same time, the future is not determined and anything could happen!
I'm hearing that you, like me, value accountability and boundaries around unacceptable/abusive behaviours - hold strong to these values 💐 As a quote I came across the other day said, "people who get upset when you set boundaries are the ones who benefited from you having none."
I agree with you @Corny that it would be great to have (and have had) more role models who set boundaries against unacceptable behaviours - this is what you're doing @Blacksheep 👏🏼👏🏼
Hoping for the best for your sister, yourself @Blacksheep, and the broader family - and sending support regardless