Sister got engaged to a neo-nazi...
I have a 1 year old who is delightful on all counts. My partners family are pretty involved and dedicated, with my in-laws driving 10 hours round trip to visit the baby about once a month. My own family are not nearly as involved. My parents have met the baby twice, and 1 time was us going to visit them. The exception, is my sister who has come to see the baby quite a bit, and is very communicative and excited by all of the pictures and updates sent to the family group chat.
Now the problem... She's been dating this guy with awful views. His Instagram is littered with politics I don't agree with, which is fine, but it also fairly regularly crosses the line. He's posted content cheering on and supporting the German neo-nazi party in their elections. He's posted content exulting aryan features. He's dabbled in orthodox christo-nationalism. He has very controlling and domineering views about women. His own family has called him a neo-nazi during a heated argument. In person, he's always kind, and polite, but online he expresses some wild beliefs.
My sister has always held a (in my opinion) naive and dangerous view of this. She insists they recognize they hold different views but they agree to disagree. She thinks she's being mature by having a serious relationship where they hold independent opposing views. I harbor concerns that this is a "nazi-bar" situation: associating with folks with these views drives away those who disagree, and attracts those who see you as tolerating it, making your network increasingly pro-nazi.
Recently they got engaged, and they're moving closer to us.
Now, I'm struggling with how to handle this with my son. I want my sister involved in his life because they do like each other, and I don't have a lot of other family on my side stepping up here. However, I'm deeply uncomfortable with the idea of my son hanging around a neo-nazi. I'd love to have my sister over for weekly family dinners now that she's closer, but I can't reasonably tell her not to bring her soon-to-be husband. We've had the conversation about her fiances views plenty of times but she always just insists they're just "being mature about it and agreeing to disagree".
I'm working through possible options here. I could try and talk to the guy directly, see if I can't make some progress towards deradicalization and possibly enrage my sister. I could ban my sister from seeing her nephew (further isolating her and cutting off a healthy relationship for him). I could demand only my sister is allowed to be around my son (and strain a lot of relationships here introducing possible future blow up). I could give her an ultimatum: prove he's deradicalizing or you can't see your nephew (how does one prove that though?).