XH and I have been divorced for 7 years, we have 2 DC, DS1(15) and DD(9).
We have a very nice relationship, we co-parent the DC, they alternate homes on a weekly basis, our houses are a 20 min walk apart and 5 mins by bus.
3 years ago, XH started seeing a woman, with 2 DDs that attended the same secondary as DS. This caused a lot of problems between him and DS, for various petty reasons, he didn't like the girls. They managed to work through it and things have gone pretty smooth since then.
Now XH and his DP want to move into his property together, his DPs daughters (15 and 13) have an every other weekend arrangement with their father, so live with their mother 90% of the time.
The issue is that XH has a 3 bed property, currently DS and DD have their own rooms but XH now wants them to share a room, so that his DPs daughters can share DDs current bedroom.
DS has flat out refused, he doesn't want to share with a 9 year old little girl and XH is refusing to back down.
I've tried to discuss solutions with XH, we could keep our current arrangement but at night, DS can just walk to my house and just sleep in his bedroom, but he's turned this into a bullfight, he wants me to back him up but I think he's in the wrong.