What do I do? I don’t want my SIL’s boyfriend to be my DD’s uncle.

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My SIL has a really messed up long term relationship with a guy my DH and I truly dislike, but put up with for her. He not only doesn’t take his medication but also uses a number of others as well as sells them. He has a lot of mental health struggles and as both me and my DH have dealt with mental health in the past and have been taking our medication we sympathize but he also doesn’t do anything to help himself. He likes to start problems on FB with people and often calls people pedophiles with no actual reasoning behind it. He threatened to take my SIL’s child from their adoptive parents, has cheated on her and multiple other issues. Usually I would say what my SIL chooses to put up with, despite having support and discussions to help her out of that situation is entirely her own choice. But she often drags myself and my DH into this stuff. I am due in a few months and am worried he is going to try to call himself my DD’s uncle, and for that I am not having it. I’m just not sure what to do about it or if I’m just being a jerk. What do I do?