Why can't young ds go in the small living room converted into a bedroom? Toddlers don't need much room. Teens do. They need somewhere they can invite their friends/gf/bf into, dance about, do homework (which at that age can need spreading out/take up lots of room).
This girl is as much part of the family as your 2 sons.
Weekend daughter could also use the small living room converted using a good quality metal action sofa bed, as they're only there weekends they don't need quite as much room.
You can get blinds for conservatories for more privacy if necessary.
Also I noticed you said I want another baby next year, how does your partner feel about that? Can you afford it? You'd likely need a bigger house.
"Why are you helping this girl defy her mum" I'm wondering this too, barely back in her life and already undermining the mum that's done the graft of raising her the last 12 years (without maintenance too?).
He has 2 children by 2 different mothers - I thought it was 4 children by 3 mothers?
Hmm yea touch of a 'crazy ex' story there, will only communicate with teachers (why? How has dp responded to communication before?), it's also rare someone does a flit like that without good reason.
I also agree another baby at this point is a terrible idea.