Family dynamic

So I am a mom of four kids, I was a teen mom. I have a huge family problem and I don't think I'm the crazy one my fiancee completely supports me in this as well. I am getting pissed about this I have a sister who is with a man who beats her. She has two kids by him I have only seen them once. This "man" also threatened to kill my children in a fit of rage when she once had left him. I gave up when she got back with him I'm over it. Plus she's not angel herself she has caused a lot of issues and was not allowed around my children for years in the past before this but I forgave and tried to move on. So now everyone in my family accepts him, whatever but it is being pushed down my throat to be around them. Which I have no desire to be. I'm pissed because I'm suppose to get over a threat against MY CHILDREN! not theirs, mine so I need to get past it. Fuck off is how I feel. I pretty much h have taken myself out of family events I have nothing for them or my family anymore it's sad because for a long time it was screw that guy I'm gonna beat his ass to now telling him he loves them. I was told I'm not being a mature adult but I am for the safety of my kids I have only met this man once btw. I don't know what he is actually capable but I have to risk potentially my kids safety and my own to be around my sister who is kinda not a good person anyways. I was told also that I should not really think he will do it because he was just mad and said it out of anger. Wtf does it matter how it was said. it was said about innocent children who have nothing to do with the situation. Plus I don't want the children being held over my head, because that is the kind of person my sister is if I were to get close to them or my kids would and it could hurt them in the long run. I don't think I am wrong and my mom called me a heartless bitch over this situation... but he also said this about your other grandkids. I was never the important one though to her. The family dynamic is screwed and I feel as if I don't have a family anymore other than my children.