speaking from experience (personal), some people cant face admitting to themselves they have made a terrible mistake, you say she left a lovely man, I stayed in a terrible relationship (and we didnt even live together) for 2 years, because I could face the fact, that I had moved from one disaster to another.
I tolerated terrible things, very similar sounding instead, an example, because I was down, my mum arranged a hair cut for me as a lovely surprise, queue a torrent of verbal (and text) abuse about what a dreadful person I was, putting myself first, what a terrible person my mother was trying to come between us, by booking a hair appointment.
When you type it out, it seems so obvious, but I was a mess (years later when I met DH who loved me warts and all, thats when I began to heal) and I just couldnt see a way out, and I wonder, given what you have said about your sister leaving a lovely man and then this, I wonder if she is in the same boat.
I was absolutely emotionally rung out, and this by a man who didnt live with me, I had the sense not to let him in a 10 mile radious of DS, I have no emotional strength at all and I really, really, really, could face being wrong (again).
I am afraid it took him to do something absolutely dreadful before I walked away, at which point, I packed any reminder, no matter how small, into black bags, drove to his house, and left it in the living room with his key.
All you can do is let your sister know you are there when she is ready, keep in touch by whatever means and wait, if anyone had tried to tell me or pushed me, I would have withdrawn even further into my shell, as it was I was ashamed and embaressed enough, without people trying to force the issue.
I rarely ever talk about this time in my life, but I hope it helps.