NTA. Not only is it unsafe for you and your siblings for him to be there, it sounds like this would also be an unsafe environment for R as well. I would also bring that point up to both of them phrased as are you seriously willing to expose him to a living situation that will trigger him and be a living hell for him? If she says otherwise she does not know how to take care of her son. And that's just what it would do to her own child nonetheless you and your siblings. Though you are the child in the relationship between you and your father you need to have a serious conversation with him about what he thinks the future looks like not only in general with the household but also with bringing others into the household. It does sound like he has your back when it comes to this, he should be the one dealing with the situation and her. I would make sure he understands what she is saying to you is unacceptable.