Eldest daughters boyfriend and middle daughter

[Emaraldisland]

I doubt anyone can help with this but just incase here goes. My eldest has been with her boyfriend for around 18 months. They seem quite serious, talking about moving in together next year.
My middle DC absolutely hates him. They had an argument going back in the summer which she's never really got over. She is quite rude to him at times, despite me telling her that's not how we treat guests. My middle DC is autistic and does struggle with her feelings and just doesn't cover up how she feels at all. To be totally honest I'm not very keen on this boyfriend either but my eldest is 20 so I can't exactly ban her from seeing him, just trying to gently persuade her not to move in with him yet.
The main issue is my middle DC and eldest's boyfriend just bicker all the time. Sometimes it's not serious but other times it really is.
I'm just at a loss for how to help them get on. If my eldest does move in with him I think her relationship with her sister will really suffer.
I obviously know siblings relationships change as they get older but my DCs were really close and since this boyfriend has come along their relationship has suffered a lot.
Do you think their relationship will improve? Amy tips on how I can get my middle DC to accept this boyfriend (particularly if eldest moves with him.

Instead of working on getting middle DC to accept him, I’d be asking your eldest to ask her BF not to argue back and wind her sister up if this is what he’s doing.
Middle DC will be juggling with her feelings towards him. Rigidity of thought may mean she can’t forgive and forget. If she was close to her sister and doesn’t like the BF that will be tough.
I wouldn’t expect them to get on. You could chat to middle DC about how he’s your sister’s choice and she doesn’t need to like him but try to tolerate for her sister’s sake.
I know you’ve asked her not to be rude to guests but it sounds like he’s not behaving like a good guest.

Have you posted about this before OP, it sounds familiar?

You can't make your daughter like her sister's boyfriend but you can tell her that it's not ok to be rude to guests in your home. What did they have a big argument about? What do they bicker about now? If the boyfriend is significantly older than your daughter, can he not just ignore her instead of bickering with her. How does your oldest daughter deal with this?