Stuck in middle of Adult DD’s

Has DD2 said why he makes her uncomfortable?
She seems very subborn. However if she feels uncomfortable, then there must be more to that story that you dont know maybe?

Maybe a blanket ban on BFs in the house. So its fair all round. Then maybe when DD2 has a fella, she will realise its not fair

Moving out time! Frankly, I’m surprised you haven’t boxed up their belongings and changed the locks. It’s sounds like a depressing way to live, like a very dull, ongoing episode of Game of Thrones with only three characters and no dragons…

This really depends on what B did, surely…?

And you don’t reason with them: this is your house; you say what goes. No negotiation because “negotiations” have been in place for a year with no result.

I agree with a pp, you're being a bit passive letting one daughter ban someone else from your home because of something they did as a teen, and you're almost encouraging them to argue when you say 'ask your sister', which implies she has the final say.

It does depend what the boy did in the past (eg if he raped someone then your sister is not unreasonable to say she doesn't want him in the house, and you should be banning him yourself). But assuming its not too bad and can be excused as a silly teen boy and he has grown up now, you need to sit them both down and say that it's your house, you get to ban or not ban and he is allowed. Agree some ground rules eg they each have a day where they can invite whoever to use the communal areas / have someone stay over etc agreed in advance so you'd daughter doesn't have to bump into someone she hates.

Also some ground rules about behaviour eg you will no longer be passing messages between them, there is to be no shouting or swearing (or whatever it is they do when they argue). Plenty of people have to love with housemates they're not keen on and get over it. See if there are common triggers like someone using someone's stuff, and figure out ways to avoid this eg cupboard each with locks

It's your house, they are adults living there because they're subsidised by you. Make that clear and if they don't like it, they are welcome to move into a flat share (where they will also have to put up with whatever partners or friends their flat mates bring home)

You can't improve their relationship but you can demand they treat each other and you / the house with respect

No more passing messages! You have been co-opted into their war.

This is your house. Stand up for yourself and YOU threaten WW3 if they both don’t pipe down.
Tell them you will not have this conflict and to not speak to you about it; they have to sort it between themselves as adults. Refuse to speak to them about complaints about one another and tell them they can move out if they do not respect your rules about conflict.
Step right back.

DD2 needs to move out and find her own place. By allowing her to dictate everything you could really damage your relationship with DD1. Fast forward to DD1 and B having a family do you want to be excluded from their lives because you sided with the unreasonable sister and her partner.
If DD2 cannot afford to move then remind her that the house is yours and if you say B can visit that is the end of it she does not get to dictate.

Is DD2s boyfriend allowed in the house? If both girls are to stay living with you then i'd bring in blanket policies. If one wants to discriminate against the other then it happens to both. So if one wants to ban friends over, then neither get friends over.

Stop allowing them to hide behind you or talk through you. They are adults. you love them both equally and will not pick sides. They need to find a way to live in the house together. If they won't sit down together then send them an email or a whattsapp chat or whatever communication tool you have that they use and you outline how it is going to be going forward. You are their mother not their referee.

How far did DD1's bf go? You say general twattiness, and that DD1 is dismissive of the reasons for dislike, but depending on what happened (was he abusive? Towards DD2 or others? Did she feel bullied by him?) I would not allow someone who has previously abused a family member into the family home where the family still lived.

Time to be the bad guy in their eyes for both girls. Family meeting. Tell dds that you alone make the rules of who can and can not come to your house. You welcomed dd2's boyfriend and will do the same for dd1's boyfriend. People do stupid things when they are young, time to move on and accept her boyfriend.
They don't have to like each other, they do need to be civil.
Putting you in the middle is also to stop. They are adults.
If either girl can not be civil then they need to move out.
You need to step up, and if you don't this will only escalate, resentment will build and it will affect your family relationships.

They need to sort it out or move out.....

I think the first poster nailed it in this case, time for them to move out.

I do think you need to make your DDs sit down together and talk about it, and if they won't tell them the situation is intolerable for you and they need to go.

Can you get all 4 to sit down together and sort this out?

They don't have to like each other but they do need to be able to tolerate being in the same house!

You have already chosen DD2 by allowing her to be like this about DD1s boyfriend. Telling DD1 she must get permission from DD2 to bring her boyfriend around. That is unfair. It is your house not DD2s. I think you need to come down hard on both of your daughters to grow the fuck up honestly.

No way would I allow any of that behaviour in my house

Ugh. Time for one or both to move out. You’ll likely all get along better when that happens.