I was like that with my violent ex. Terrified of being alone, I felt very rejected as a child and still took rejection badly then (met my ex when I was 18), and was afraid of ex going of the rails and seriously hurting himself in some way. I felt like if that happened, or our family split, it would be my fault for not fixing things/keeping it together. He was good with our son, never controlling (or so i though, he was in subtle ways), and I felt so sorry for him, even when he hurt me. I knew he was seriously mentally ill, but he was so good at hiding it from others, and the more normal the abnormal behaviour became, the more numb I got to it. The more I thought "if he could just control his temper, everything would be perfect". We argued a lot, but he usually ended up throwing or breaking stuff, if I went too far or said the wrong aka totally acuratething. Put his elbow through a window, foot through TV, pushed a bookcase over, fist through doors etc... He kicked me in the face at one point, and we made up a few hours later. When we finally split and I threw him out, I had such awful flash backs about that (and other incidents I'd really repressed, the look of pure hatred and malice in his eyes etc). At the time I kept telling myself he wasn't that bad etc. The more crazy he acted, the more I tried to get him to face reality, which would hurt his feelings, then the more guilty I felt, the more I tried to be nice, and understanding, and the more he walked all over me, repeat, repeat. Then he was caught kissing someone else, who admitted it had happened, which was the final straw (which seems trivial compared to everything else he did, but it made something click in my head at last. Up until this point, I'd suspected he'd been cheating all along, but had no proof).
No idea what's going on with your sis, but I hope that helped explain why someone might stay, crazy at it seems. For me it was a mix of low self esteem, fear of rejection, people pleasing, manipulation, gaslighting, and trauma. And he did the classic, came after me when I moved on thing, a week after I kicked him out, beat the crap out of me while he was stocious. Had had enough at that point, and took him to court. He got off, and now I have a caution on my record for assaulting him (long story). I hope your sis is ok.