Future MIL invited toxic cousin to be groomsman on our behalf

Need some advice on this. My future in-laws and I have had a pretty bad relationship. They have been manipulative, verbally abusive to me. A LOT of this started because of rumors that his cousins started, after fiancé accidentally made them feel less important than his best man/best friend, so they blamed me for his "faulty priorities". They threatened and harrassed me and convinced MIL that I am a bad person so she uninvited me from holiday events after I already flew out to be with them and had nowhere to go. They said if you tell anyone about this you will be responsible for making everyone's health bad due to heartbreak, and the elderly will probably be hospitalized. Just...terrible stuff like that.

Even now a year later people haven't made things right. They admitted to him they acted irrationally but they're still cruel to me. It's my MIL's family (her sisters, her nephews and nieces) so she's always gaslighting us to defend them.

One of these cousins is like a brother to fiancé. We made it clear to the MIL that we'd still love to invite the cousin and resolve everything if the cousin just makes amends.

Now we find out that my MIL asked the cousin to be a groomsman... She told him that my fiancé really wanted this, and the cousin is apparently really excited and told everyone in the whole family. But he still hasn't actually reached out at all ever since he promised months ago that he will apologize for the rumors, threats, and harrassment. MIL reminds me every 2 seconds that they are like brothers and now I'm family, so the cousin is my family too. That I need to "choose happiness" instead of still being triggered.

I'm not really sure what to do. They've made the year hellish for us, and I feel anxious trying to decide what to do with this information. I honestly didn't even want the cousin to come, nevermind be a groomsman and participate in our cultural wedding events, be in all the wedding party photos, etc. In my culture the groomsmen give offerings to bridesmaid to symbolize trust right before the wedding, and my anxious-superstitious self feels paranoid that someone bad is going to be part of that if we don't do something about it.

EDIT: I'll admit that my fiancé's response to all the abuse was not as firm or immediate as I had wanted when it first happened and it caused a ton of problems for us. I felt really betrayed by them and of course, disrespected by him when he chose to always be passive rather than actively resolve the issues with his family.

It's gotten better since then. Besides the confrontation where he laid out what they did, how it affected us as a couple and me as a human being, and told them to make amends, he has had almost no contact with the cousins for half a year. Since fiancé moved out, sometimes MIL forces us to interact with the cousins by driving them hours to see without notice. This felt like a huge ambush, so after the initial shock and a lot of push, he told them they have to make other sleeping arrangements.

Also no contact with the aunt who said unspeakable things in a year. She asked him not to tell her daughters (who were uninvolved in all of this) what she said to me, but he told her he will tell the truth if they approach and ask why. We've told MIL there is just 0 chance or reconciliation every time they've asked or try to assume the aunt is invited to our wedding.

In terms of his mom...it is up and down. She does "try" to be better (according to her version of this) but so easily slips into gaslighting, overcontrolling mode so all progress feels lost. MIL receives A LOT of positive reinforcement from other family members because she is the breadwinner of the family and the youngest. For the most part, fiancé and I keep to my side of the family, and we've spent all holidays with mine since this happened.

I know he is growing but I've communicated what my limits are. Can't control his family but he can control how he responds, so I've told him under which circumstances this stops being worth it and I'll leave...

UPDATE: Fiancé has sent a message directly to the cousin about not being a groomsman and to his mom about not overstepping. He also privately messaged his dad and told him to stop enabling mom's behavior like everyone else. She doesn't need more people telling her she did the right thing. No response yet.

Also he finally agreed to go to therapy!!!! I've been going by myself for awhile but he set an appointment for this upcoming week. I'm frustrated it took this long but hopefully this helps him and also helps us repair our relationship moving forward.