Unhealthy Relationship Dynamic

My youngest sister has become involved with a man who manipulated and guilted her into compromising her values and morals to engage in an affair with him.

When he had the encouragement and advice to come clean and be honest with his wife about the affair so that he could go about things the honorable and respectful way, he chose to instead manipulate my sister by trying to make her feel as if she didn't really love him if she wasn't going to just say "f*ck everything and everyone and just be happy".

My sister was concerned with his wife and their 2 kids and how they would be hurt and affected and he was not.

Fast forward to now. My sister and this man have a one year old and I have had to step in as a second parent to fulfill his responsibility because he never wanted kids and despite now having 3, he either avoids them at all cost leaving them all in our care and responsibility or he is extremely aggressive and mean to them, once even screaming at the top of his lungs to his 11 year old that she was stupid and that he mom was a b*tch.

Now that it is clear that he has a lot of issues. My sister has become fed up. She loves him but simply cannot continue this pattern of unhealthiness. She informed him that she needed him to get help or see a counselor and now for 2 months he has been elaborately lying about seeing someone.

I told her that the easiest way to sus out the truth from him was to ask for his doctors name or to ask where he is treated.