If you had his kids all the time, I would disagree with you and say the two oldest should share the biggest room, as they are close in age.  The 6 year old is 8 years younger and that is a significant difference.  However, if his daughters are only visiting four days a month then yes they should share.  I can sort of see your DH's argument, as your son would only be inconvenienced four days out of the month.  However, I can't imagine his 6 year old will want to share a room with a baby.  Also, you need to remind your DH that his youngest probably has toys and things that are unsafe for a small baby.  If his kids are only coming 4 days a month, then I really don't see why this is a big issue.  I think sometimes it becomes more about a power struggle than about the actual room assignments.

Non FTR kids do not get priority on rooms.  Configure one as the guest room and they can share it EOWE.

Your DH is afraid if he doesn’t give his kids what they want, they won’t come over, which is probably a realistic fear, especially if they are not happy about the new baby. 

Why not pull the two teens into the discussion and see what they say? 

No offense, OP, but you probably should have discussed this sooner. 

ETA: I thought his kids were the 14 and 16 year-olds - hence my thought of them not coming over - I don't think 6-year-olds get to decide that.

I'm a big believer in not making kids feel like guests in their parent's home when it's avoidable. If they get treated like guests then they are going to act like guests. It's unfair to treat them like guests then expect them to act like members of the household.

I say put your baby and YSD in the biggest bedroom. The teenagers can have their own rooms, and your DH will have to be dilligent about YSD's toys being put away and kept from baby (which you all would have to be anyway).

Baby isn't going to be super independent and playing solely in their room for, what, 2 years? By that time, your DD will be going off to college or moving out on your own. Then, the teens can share a room on the rare weekends that they come over (because my guess is that OSD will stop once she has a license). At that point, you can buy them a lofted bunk bed set, move YSD into her own room that will be used on off weekends as a guest room, and Baby will have his own room just as he needs the play space.

There is no ideal situation because you don't have enough space for 6 people without someone sharing. This plan means everyone shares at some point. Putting a 6 and 14 YO together is just going to lead to fighting in your home, so keep that part in mind, too. The 6 YO is going to be the most flexible in all of this, so put her and baby together.

The AAP recommends that babies sleep in the parents' room for at least 6 months and preferably a year. Is that part of your plan?  If so, that buys you some time.  You could put baby's stuff and the 6 year old together, but the 6 yo would not be disturbed by a crying infant in her room at night.  

I would not want to put a newborn in the room with a 6 year old, but I could see having an older baby in the room with a 7 year old, especially if it's only 4 days a month.

I would discuss the room situation with the 2 older girls, not to let them decide but to get their thoughts and suggestions.  Maybe you'll find out that your DD is willing to take one for the team and bunk with a 6 yo 4 days a month.

And this is why I wouldn't bring a child into stephell. Right now I get up and leave when I dont like the situation. I leave the room, I leabe the house, or I could leave the marraige if I choose. No bios to tie me down. My bios are grown.

Although I will tell you my ex did not set up my DCs room till I was in the hospital.  I was ready to scream. 

Are you taking some time off from work when baby is first born?  I agree with PP, it may be best for baby to sleep in bassinet in your room at first.  It seems that there may be only a year between when baby goes into his room, and oldest goes to college.  I would think about that. 

 Would have the 14 and 16 yr old share. They are the closest in age and it's only 4 days a month. 

and we’re expecting our 2nd in the new year. Our 1st child together is 3. We have a 3 bedroom house so someone is going to share. Even though the baby will be sleeping in our room for the first few months, we’ve decided to put the baby with DD3 and have SD keep her bigger room. We figured she’d be off to college in the next 5ish years anyway and then we’d move DD to that room and figured SD can sleep there when she visits from college. SD didn’t seem too crazy about that fact but I think it’s unrealistic for a kid to expect that their room to is going to stay a shrine to them after they leave for college. Especially when other siblings would be sharing rooms. 

So what is my suggestion? Three options (that have probably already been said:  

1. Two oldest share and 6yo and baby get own rooms

2. SDs share and then DD and baby get own rooms

3. Oldest get own rooms and 6yo and baby share

4. Oldest kids share AND 6yo and baby share and other room can be a toy/entertainment room. 

Hopefully I didn’t misread the number of rooms. I have yet to drink any coffee this morning. 

I would not put a six year old with a new born infant, nor would I put a six year old and a fourteen year old together .

That leaves the sixteen year old and fourteen year old together. It's a lousy four nights a month. The room would be BD16's room the huge majority of the month. The 14 year old really cant have all that much stuff in her room if she only comes short a short time, pus some of it can go be in the six year old's room as that child doesn't need tons of space if she is only there also four days a month---so, in a way both the 16 year old and the six year old end up sharing space with the 14 year old. All of the girls should 'share in the pain' of adding a new little member to the family--- it's not like you're going to send baby back because none of them want to be the one to be unconvinced. 

I get you want the baby, to have his nursery. Whatever rooms are what, the nursery should be the closest room to your bedroom. Your DH is being a pouty jerk. Tell him he has to help get the rooms ready and the two oldest girls can get their rears in gear and help him. 

Does your home have a basement?

Your kids can go down there??? Dh's daughter, babies nursery and parent room using 3 bedrooms?

I knew a bm who put her step child in the basement because of eow and moved her kids into the step childs 'childhood' bedroom. Nice, huh? NOT

Newborn should be with you and dh.  OR put a single bed in the 'nursery' for you and baby. All kids bunk together. 2 bunk bed sets will fix THIS problem. Years ago to have 4 kids in 1 bedroom was not that far fetched.

 Hopefully you have a basement--or bonus room/family room.

Another option and the one were we're going to do if we had won custody of SS14- put barn doors on our formal dining room and turn it to a bedroom. If you don't have a basement or bonus room, are there any formal rooms that could be converted?

teens share for the four days a month. I cannot imagine the hell that will reign down if the 6 year old were to get into the older child's makeup, clothes, electronics (or anything else). Actually, yes I can. Because we had four girls in our house. The middle two shared by choice. Son took the family room and the other two girls had separate bedrooms. The youngest is 7 years younger than the oldest and the two closest in age haven't gotten along since the youngest was born (and they are only 16 months apart lol). The six year old will be there the longest with the exception of the baby and isn't likely to change bedrooms so she should have her own space, the baby will have his own space and the two that are close to launch date can deal with four days a month of sharing. 

Is there any chance you can finish out part of the basement so that one of your teens can have her own space?

If not (for whatever reason), my first gut reaction was to have SDs share a room since they are used to one another. In two years, when DD16 leaves for college, you can swap the rooms around.