Not sure if this is the right place to post this, but I could really use some advice.

My sister has been in an on-and-off relationship for the past 2-3 years. The boyfriend seems to be using her for monetary and material reasons and is very manipulative. He tells her that he has spiritual visions of my dead father. I find this extremely offensive. When it comes to my dad, my sister is totally gullible. She truly believes her boyfriend communicates with my dad. This man has never even met my dad. I think my sister has guilt issues about my dad bec when he was in a coma and we were caring for him, she suddenly left for months without reason. He has also treated her badly. This is usually the reasons why they breakup. He cheats on her. He has never publicly acknowledged her as his girlfriend on his social media and has refused to do so. He has very poor communication with her. He doesn't call or text when he says he's going to. And on several occasions, he has stood her up. On this most recent breakup, she caught him in a lie. He then proceeded to admit to her that there was someone else and that he could not resist the temptation of this girl. My sister begged him to choose her over the other person. He chose the other girl instead. With each breakup, my sister reacts more and more dramatic. The breakup before this most recent one, she changed her phone number and mailed back all his things, only to give him her phone number a few weeks later. This time, she suddenly decided to move! She said her apartment was haunted and tainted with his memory and that she couldn't breathe in it. She wrote dramatic posts on her social media accounts about what he did to her and how she was really done this time. And, yes, she moved. She moved to a small yacht. She knows nothing about boats. And she made a big deal about this fresh start. This breakup lasted longer than the others, 4 months to be exact. I actually thougt it might be really over this time. Then, 2 weeks ago, the boyfriend calls. And just like that, he's back in. The thing that worries me is how easily she accpts him back. Her opinion of him changes in the blink of an eye. Suddenly, he can do no wrong and he is the love of her life again. She suddenly forgets all the things he did and all the things she said and did. And with that, she expects the same from me. She wants me to accept him and their relationship. And if I don't, she blames me for potentially destroying our family. She can't understand why I don't accept or like him. It is actually kind of scary how quickly her attitude changes. She suddenly attacks me and says I'm the reason for any drama and negativity. I really don't know how to deal with this anymore. After every breakup, I've been supportive and always there for her. I've never told her what to do and not to be with him. I've always just been supportive, always said her decisions are up to her. But this time, I feel that I can't do this anymore. I almost feel like I'm dealing with a drug addict. What is the proper way to deal with someone like this? Should I just continue to be supportive or should I stand my ground this time and keep my distance and no longer drown in their toxicity? She's my sister and I love her but I have had enough of both of their behavior. I cannot handle her cyclical relationship or drastic behavior any longer.