Don’t know how to feel when she keeps jumping in the fire

So long story short my sister is in and out of an abusive relationship. She was serious this time and actually moved back home for a while. She said he’s changing and was wanting to go back and her daughter wants to go back but it’s so hard to keep being there for it. Not that it compares but it’s almost like a drug addict. The more they go back the harder it is to be there and support them. It isn’t their fault bc it’s a disease or in this case it’s hard but at some point it is their choice to go back. She wanted to go back but was worried about what everyone thought. Well now she’s pregnant. This is the worst possible time for this especially when their first kid is what brought on most of the problems, they were just not ready for it. Idk how to feel. Obviously I’m there for her bc I’m the only one who knows rn but im so incredibly upset with her. She swears up and down it was an accident but the story is so specific “she had a cramp and had to move as he was finishing so he couldn’t pull out in time bc she moved unexpectedly” and they way she talks like “ I just hope it will be better this time” makes me question hard core. This has put so much stress and strain on my family and I just don’t know what to do. He doesn’t really hit her he just breaks things so far but that’s the start of it. In the last fight where she moved back he slammed a door trying to get away from her but she was following him and he hit her hand and broke the glass cup in her hand. I know it’s not all about me and she is obviously the one who should be upset but I just don’t know what to do. It’s so hard to be there for her when she keeps going back to a shitty situation. Idk I feel selfish for having a hard time but god this is just going to be so crazy. I just don’t know what to do.