Does the trip happen on a SS weekend?  If so, he should probably go if you are bringing the other child in the family. He is part of the family unit.. at the very least on the custody time with his dad.

If it's not his weekend.. then I think you are fine to take a trip and bring your DD.

If it is though  I think I would explore having DD stay with the grandparents again.. they likely will be happy to do it both times.. and just make a point of not asking again for a while after.

Tell him that it's your birthday weekend and that you want to take your daughter. Taking SS too makes it into a kiddie's weekend. He can take his son when it's his turn (you don't have to tell him it's going to be kiddie free - save that nugget for later). And when the time comes,  make it so he doesn't want his son there (by letting him know you're buying lingerie, toys, etc. ). The kid will have lots of other trips with his dad and doesn't have to be on every one of them.

It's certainly understandable that you would find a vacation with your DD much less stressful than a vacation with SS who has behavior problems and a BM that you have had issues with. I don't think the feelings are "selfish" at all.

I feel the same way about kids in the bedroom. I don't mind my daughter in my room nearly as much as i mind my SO's sons. However, if we move in together, i'll insist that there be no kids in the bedroom. I see that as a compromise.

Vacations....that's a sort of similar issue. I have so many fond memories of vacations with my kids. Vacations with some of SO's kids are a form of torture, though. I kind of hate the thought that there can be no vacations with my kids without them (well, some of them anyway) too. That's a tough one. My kids are much older than yours so i wouldn't be giving up near as much. Just offering sympathy amd understanding. 

Be honest with your DH:

"DH, I'm hesitant about SS going. When we take him places like this, he <insert behavior you described above>. That is stressful, and I'd rather not have that for my birthday. If you're not comfortable with leaving him behind, or going on a weekend that he isn't here, then I recommend asking your parents to keep DD. If that can't be arranged, I'd rather not go."

If DH protests, tell him that you'll only consider going with SS if DH can teach SS to behave appropriately over the next few months AND starts taking his phone when SS visits. If he can't do that (which should be a pretty easy parenting flex), then he doesn't go.

I understand you don't want to ask the in-laws a second time, but if your DH really wants to plan a trip FOR YOU, then he needs to think it through all the way and make it FOR YOU. He shouldn't have promised a chalet without having considered childcare. He shouldn't have looked into a birthday present that left you still having to be "on" as Mom (because a birthday weekend with a hot tub at a chalet with a 6 year old doesn't sound relaxing). He should have read the situation and seen that you have stepped back from SS and calculated that into his math.

He didn't, so put it back on him to either fix or scrap this gift. I understand that "it's the thought that counts" but this sounds more like someone who gives their friend a wool sweater for Christmas when they know their friend is allergic to wool. It's not thoughtful to give someone something they can't reasonably like or use. Then you're just giving a gift to make yourself feel better (or to give yourself a gift disguised as a gift for someone else).

If it is not a skid weekend with you, then no need to bring skid, he is with his mother and you are not going to change weekends.

Hence book the weekend when he is suppose to be with BM.

If your husband books a weekend he is with you, simply ask the grand parents to babysit both kids, both kids are their grandkids, and then no kid taggs along.

You have to decide, if this is a naughty break away, meaning adults weekend or simply a celebation of your birthday.

From experience I can say, both kids are minors, very young, either they both go or they both stay... it is the sad sad life of blended families,

Wait, it's your birthday trip, right? Then you decide who goes. SS is not your child. Why do we need to give  long winded expanations to SS about why he doesn't go on a trip for your birthday and DD does?  That's part of blended families.  You are DD's mom, so she gets to go. Simple.  

I ditto the two above - first, why not get a sitter? I'd much rather go alone with DH than with a 6 year old. Second, it's your birthday, your choice. DH can plan his own birthday trip if he wants SS to go somewhere. 

I would be really annoyed if my DH wanted to bring skid on MY birthday trip. If I want to bring my kid I should be able to do that without having to bring skid too.  Skid has his mom to take him on trips too. I never make my kids miss out on anything just because of skid. It's not my responsibility to make sure skid has double vacations. Ops dh should try to arrange it when skid isn't there. Another way SMS get screwed in what should be a fun, anticipatory time. 

"First he said shall we take dd6."

You know, as a lead-in, that phrase was obviously designed to manipulate. It opened the door to him bringing his kid too. I would tell him that you had thought it over and decided that you'd rather have a couple's weekend than a kiddies one. Does your DD ever do sleepovers? Are there any parents who might take her for the weekend with the understanding that you'd owe them one?