LOL at people suggesting three people should be crammed together in one room so that precious SD doesn't have to share four nights a month.
Nothing surprising for MN but still entertaining.
Meanwhile, in the real world... Yes, she will have to share. At least between the time baby has outgrown being with you, OP, until such a time as you can afford a bigger house.
I feel like it's a very MN thing to be precious about sharing rooms. I shared with my brother until I was 13/14 (he is 6 years younger), I only stopped sharing because my older brother left home. Guess what, nobody died and nobody was traumatised.
In this case it's not even full time. Yes, I understand SD may have complex feelings about being pushed out but that's for her dad to discuss with her at length so she understands the constraints of reality.
And the baby absolutely should have part of the room decorated for them. I sniggered when someone suggested the OP should put SD first - before her own baby - wtf ?!! Whatever happened to treating all children fairly ? Well, only when it advantages SD I guess.
OP, you're going to have to reorganize that room to accomodate both SD and baby. I would suggest doing it with SD if she is amenable - this is the opportunity for her to have a voice about how to do this in a way that's least disruptive to all involved. Obviously, not if she is stropping and refusing to be involved, but if she shows a minimum of good grace that's the opportunity for her to also redecorate how she likes her half of the room.