To not go on holiday with stepchild?

Depends on the holiday your DH is suggesting. If it's a couple of days in the UK then maybe I would say yes, if you don't want to use up annual leave DH could go without you.
I think it would be a nice thing to spoil DSS a little while the other child is on such a special trip. But I wouldn't agree to a 'proper' holiday either without both children.
We've had all this too - except it was the older SC who got to go on expensive holidays with their Mum, we didn't immediately take DD away so she wouldn't miss out but we did take the opportunity to do things with her that the older kids probably wouldn't have enjoyed so much or that we couldn't afford to do with 3 children in tow.
So I think YABU to say a flat no. But it really depends on what's being suggested here..

Sounds like DH wants you to do the parenting work for his son - otherwise why would he not grab the opportunity to have 1:1 time with DSS? Is he worried he won't be able to manage by himself?
The other thing that occurred to me is that it would be a great time for you and DH to have an adults mini-break without any kids, and then take both the boys on holiday together later. But I'm with you OP, I wouldn't want to go on holiday with someone else's kid and not my own.

It might be how it is with blended families but it’s still going to be hard for a 12 year old with an excited younger sibling talking lots about their fab USA trip.

We have a blended family and it’s my oldest that gets to do more as she holidays with her dad and his new family sometimes (and sometimes not and that’s ok too)

We have on occasion done a smaller trip with our youngest whilst she’s away - something that the eldest wouldn’t enjoy so she wouldn’t feel she’s missing out.

If it was me I’d have a few days away doing something more grown up with your DSS. (Assuming you can still do a family holiday all together as well)

What about DH takes him away and they get some good one on one time? That feels like a good compromise to me between your position and his.

YANBU.

It's actually nothing to do with DSS. Your child's grandparents are treating them to a holiday, DSS's grandparents could also do the same.

It would be an absolute non starter for me, I wouldn't agree to a holiday without my child to "even things up". If your partner wants to do something he can take his son away and no you absolutely shouldn't be forced to partake in petty poker scoring games. Ugh.

Op - the people who are saying it's ridiculous have misread your op, because it's slightly ambiguous.

They think you mean - joint child PLUS one step child is going and one step child not.

Not just joint child.

I can also empathise with a 5 year old finding out you all went on hols without them in their absence (even after a trip to Disney, they can be pretty brutal 😂) I would defo plan something nice for the four of you together then later on in the year, something nice for DSS to look forward to.

Would your DH agree to you both taking your child away whenever his other child has a holiday with the mothers side of the family? What has happened previously?

I always feel sorry for "second" family children on here. It's like they can never have anything nice or just for them.

Your DH can't give his son a Disney trip, but he can give him his time.

Why doesn't he arrange some father-son bonding activities and really focus on his eldest?

It must be difficult for your DSS to see his dad living full time with his other child but not him, now your DH can give some one-on-one time with his eldest son; he should be grabbing that opportunity and making the most of it.

Soon DSS will be in the grumpy teen phase where he won't want to holiday with you 😂

I don’t see the problem to be honest, if your child is going on holiday with grandparents to be spoiled rotten, why can’t you and DH take you DSS away? It would give you some one on one time that I assume they haven’t had for quite a long time if your joint child is 5, they are also quite a bit older so would get to enjoy more activities too that they are probably more limited on normally.

you refusing on the grounds that you wouldn’t enjoy it to be honest sounds quite selfish to me, and unfair on your DH and DSS, what did you do before you had your 5 year old?

if DSS step son went to Disney with his mum are you honestly saying that you & DH wouldn’t take your joint child away at the same time on principle because ‘it’s not fair’, I can’t see the problem if it was that way round either, it’s a good opportunity for some one on one time, if you normally all holiday together, one time every few years wouldn’t make any difference whatsoever.

the only way it would be unfair is if you used the opportunity of one of them already going away to have a big ticket holiday that the the other would prefer to the holiday they are going on.. but that’s not the case here.

As a former step child it’s just the way in blended families, my dad took me on holidays my step brothers missed out on, just like their mum took them on holidays I missed.

It’s only unfair if the family unit excludes kids, ie if you took your son with your DH and not his child. When it comes to the wider families it’s luck of the draw, just like some kids in their classes might go Disney and they won’t. Different families have different experiences and that’s life.

Ah okay fair enough. Different strokes for different folks and all that.

I suppose because she is the child’s step mum?

I would defo do something special with DSS, sounds like a perfect opportunity! You DC is already having a lovely time with your parents on a holiday of a lifetime, why not treat your DSS. If your DSS was on a holiday of a lifetime with his mum, would you not want to do something special with your DC? I feel so sorry for Step kids on this forum 😢

I agree. It is really sad for the son.

I might be wrong but I am getting the impression the op doesn’t really like the SS?

I think it’s unpleasant not to compromise and do one big treat day as a three. And then dad/son to go off and do something else together. Camping or hotel stay or something!

Poor DSS.

He needs to step up and take his son away if he's so bothered.

Why does he need you there, is it to make it more fun or he'll be bored with just his son for company.

He should welcoming this time spent with his older child.

Don't let him guilt trip you in to anything.

Your idea is the one that makes sense. Book a family holiday for all 4 of you when your Dc is back.
If your dh wants to spoil ds then he should. He could take him away for a weekend, maybe a theme park if he would enjoy that or whatever else would be fun for ds.

She’s not stopping him doing anything he likes with his son. He just doesn’t want to do it without her. How sad for his son.

YANBU

As you say he's going with your parents, who probably don't have the same relationship with DSS. Therefore can't be helped.

It sounds like DH feels guilty his son isn't going so is trying to compensate. But it DSS went with mums family he probably wouldn't do the same with DC.

I would just say no, you aren't holidaying without DC and explain that it's the same when DSS goes with mums family etc, he can take him if he wants but you won't be going.

Be honest with yourself, if it was your child left at home while the step child went on a once in a lifetime’s holiday- what would you do?
Do nothing and carry on as normal?
Have a couple of treat days out and spoil him a bit?

Come on, you’d do something!

But in the same way it would then be hard for OP's child to hear they were then excluded from a family holiday as well. It wouldn't be fair on OP's child. It's fairer on the SS as he also holidays with his mum's side of the family by himself.

So he feels bad that one of his sons is going to Disney and the other one isnt..

Well that's life in blended families. I don't know how close you are to dss mother but I personally wouldn't be at all comfortable for my child to go abroad with their half siblings gps. In fact, I wouldn't agree to it.

He is entirely unreasonable and bordering on absurd to feel your parents should be talking dss too. I can only see in cases where the step child lives with you full time or/and no contact with other parent and side of that family where leaving the child out would be wrong.