What to do about my (18) brother(23) and his toxic fiance (22)

Hey all. So my brother and his fiance have been together for around 7 years now. To be honest we didn't start off great to begin with, my family and I were really nice with her and tried to treat her like part of the family. But she was just weird with us, she didn't talk to us or eat in front of us. If we went out for dinner she demanded that her and my brother have a separate table away from the rest of the family. She didn't let him go on vacation with us.
She did a lot of different **** over the years, if I was talking to them and she wanted to eat they would just kick me out of the room. When our parents were away she purposefully left a mess that I would have to clean up. Planned her birthday party on the day of my birthday so he couldn't go. Rummaging through my room while I was gone. Petty stuff like that. But what was worse was what she's done to my brother, when he got with her he had a lot of friends, he's always been a happy go lucky, outgoing person. But she stopped letting him see his friends and going to family get togethers. (My other brother and sister live in a different city so we don't see them often)
About two years ago they both moved in together in a city 3 hours away for university. We see him once every one or two months now which is okay. But she doesn't let him go out or have friends, which just seems so bizarre to me, I feel like he should be enjoying himself in university, but she just has him cleaning the apartment all the time.
The latest thing that's happened was that my sister (27) had an engagement party. My brother's fiance came out with this demand that either they go home with her friend before the party or that my parents drive them home (again, 6 hour drive all in all) afterwards. Oh, and she wasn't even going to the party, just him. Basically just she wasn't letting him go, she wanted to ruin the party for my sister.
He ended up going regardless and as punishment, she ignored him on the phone for two days, and that he was allowed to come home when she was away to work.
Another thing is that she's been trying to plan their wedding, even though he's expressed openly that he doesn't want to get married yet. She texted my mum from his phone (he's dyslexic and doesn't type the way she did in the text) demanding that my parents would have to give them ?4,000 for the wedding.
Just to clarify, this isn't some petty thing just because he's my brother. I adore my other brother's boyfriend and sister's fiance, they fit so well into the family. My family have tried so hard to make my brother's fiance fit in and feel comfortable.
I don't know what to do now, he reconciled with her as soon as he went home. I don't want to associate with either of them anymore, I just feel like they bring in too much stress. Is it childish for me to just stop talking to them? The thing that pisses me off most is the engagement thing, my sister has gone through a lot and is the one person who deserves to be happy, it hurts to know that my brother's fiance wanted to ruin that for her.
TLDR: Brother's fiance controls his every move, including which friends and family he sees. Didn't let my brother go to sister's engagement party. Brother is too immature and doesn't stand up for himself. I need some advice on how to deal with them.
Sorry for the super long post but I feel like you needed that context ? Thanks in advance!
Edit: Thank you all so much for your advice and responses. I'm sorry to hear that some of you have been through something similar.
Just to explain a little more, my family didn't just lie back and let this happen, they've tried so many times to help and make him see that this behaviour isn't normal but I think he's too far deep.
He's told us that he isn't happy and I can tell he hasn't been happy for a while. He's tried to get her to change mutliple times, even asked her to go to therapy but she always gets around him again.


Source.