AITAH For Wanting to Cut Off my Relationship With my Sister?

My girlfriend and I have been in a relationship for nearly 3 years, and while it has been fantastic, there's been one major issue; I had told my parents that she had been abused as a child, and well my family had some issues with it. In short, it blew up in our faces. Basically what had occurred was during our trip together, my sister had messaged me telling me to be careful of her, later accusing her of being manipulative, abusive and controlling (despite never meeting her). She even tried using her psych major as the defacto "I say this, she is that" sorta thing. I had ignored her messages, and later she messaged asking if I was ignoring her, or if my girlfriend was preventing me from responding (she wasn't). We understand that their concerns are valid, of course there are merits to when one is abused, and could result in a person with "red-flags"; the words my family had continued to use even after telling them that I understand. However, they just kept pilling it on. But while I had fixed and gotten my parents to apologize for their behavior, my sister is the only one that refuses to accept that her behavior was wrong, and to apologize for it. So, my girlfriend and I had agreed that if nothing got resolved, we would break up. I wont get into the details as to why, but I offered to cut off my sister so I wouldn't have to break it off with an otherwise fantastic, sweet girl. She isn't sure what to make of my offer because she feels guilty for potentially hurting my relationship with my sister, AITAH for wanting to cut off my sister for how she had acted, and continues to act towards my partner?