To want to do something with just my kids?

Frankly at 16 it’s beyond belief that she would give a single shit about a holiday with her step mum and two younger siblings. If you took her she would be miserable and it sounds like her mum’s just stirring the pot for her own sake.

As someone with a step parent. Ive never forgot the time my dad refused me something because my step"mum" told him her sons wouldn't get it. And I dont remember much from my childhood but this has always stuck with me. My mums passed away and I thought they would get it at their dad's if she said no.
I say go with your kids. Unfortunately it's part of a blended family, each family is different and do different things in each house.

I never knew they were unhappy so I was fine and really happy! Honestly as an adult myself now, I think a lot of their difficulties were generational (eg many people that age were not raised to communicate effectively) / personality so divorce wouldn't have solved that problem at all. They weren't a great marital couple but they were the best parents.

Did you really never feel sad for your parents, knowing they were spending their life being unhappy?
Or unsure about the future, worrying if they would crack one day and end it?
I'm intrigued as usually posters comment about the sheer relief of the black cloud of their parents unhappiness lifting, yours is a new perspective.

Don't really have comment on OP's dilemma but this makes me so glad my parents knuckled through and kept us as a nuclear family unit despite wanting to divorce.

That ensured I never had to feel like the dirty outcast of the family – I know that's not OP's intention or meaning, but kids take things differently, and carry that feeling for the rest of their life!

I know it's never clear what the best thing to do is (should unhappy parents divorce, after divorce how much do you treat your stepkid like your own kid), but in my particular case I'm just SO glad things went a certain way and I always felt unconditionally, without question, belonged and included.

Again not a commentary on OP's situation. Just a selfish little comment about myself.

Good that you have done it guilt free. You should be able to do things with only your children without having to justify that. She has her own mum to do things with. Her dad isn't going along so it's not like she is left out of a family holiday. Its good that you set the boundary as well with everyone concerned as you don't need to explain your choices to anyone.

It's a few days in Spain,they don't keep her in the coal cellar.

Oh give over.

She's not spilt rice and corn on the floor and ordered her step daughter to seperate them out before feeding her.

It’s 3-4 days away in Spain? Not the whole 6 weeks.

I do wonder if all of this has come about because one of the kids told DSD about it and she assumed her dad would be also going on the trip? So it was probably right of DSDs mum to ask if they were taking her too, or assume that if it was in their custody time that they would be taking her.

if not, it seems really odd to me that DSDs mum would actually want or expect her ex’s wife to take her away with them, but who knows??

I don’t know what attitude people are talking about, either. She is putting her 2 children first by ensuring that they have some special time with her and she’s understandably not ashamed of that. It’s important for children to have quality time with their mum without other people being there. OP’s husband isn’t going to be there, dsd isn’t going to be there, so the dynamic will be very much focused on OP and her children together. Whoever begrudges that is being really unfair.

People have taken that the wrong way and can’t hack that a mum who is also a stepmum would dare to focus on her own children every once in a while without a shred of guilt.

Would a 16 yo really want to go away for 6 wks with younger kids missing her summer with friends.

Sounds like the bio mum is stirring shit.

Sorry but I feel that leaving her behind would be unkind and would only reinforce the evil stepmother narrative both to her and her mother.

I would possibly agree with you if you'd only been with her Dad a couple of years but your children have grown up together and you've pretty much known her all of her life. This would single her out as not being part of the family.

Not sure what "attitude" people are referring to.
OP hasn't said a bad word about her DSD, said she's lovely and also will be providing for her to a degree via the family pot.
All snippiness (which has been pretty mild actually) seems to be in response to posters making out that her children don't have a right to private special time with their mother. Which I would get pretty snippy about too, to be honest.

Need to put on a jumper, it’s so cold in here.

your decision to holiday with your kids - fine
your attitude towards your dsd - stinks

Hope she’s successful in life in spite of you

Because they aren't all the time. Step child doesn't live there 50/50. She isn't excluded when she is there and she is going on holiday with OP, her father and siblings later in the year.
The step daughter is 16. Even families who live together 100% of the time can have holidays without a 16 year old.
It's not like she's a 10 year old.

I was on your side at the beginning but some of your more recent updates...not sure, your overall attitude towards your DSD seems a bit off

18 year olds are not children

How would your children feel if one of them went and one was kept home?

The absurdity of some of these posts.
If a child goes away to University are the remaining family not a unit that carry on and do things together, weekends away, meals out etc.,?
Honestly OP, drop the rope on the madness of engaging with some of these posts.

But 1 to 1 also has to be equal each child having a fair turn

True, however, age/logistics play a big part. You cannot, for example, take a 5yo to a see 12+ film at a cinema. But that's not a fair reason to exclude the 12yo from a family trip to the zoo.

I believe when you take on a relationship where children are involved you have to unconditionally treat the children with love compassion and equally you can't tell any child they are lesser no matter what the relationship it's not their fault they are dragged into a situation

But 1 to 1 also has to be equal each child having a fair turn

OP should be able to do things with and for her dc. This includes taking them on a holiday and slowing down to spend some much needed time together just them.
Why are some people choosing to ignore how important this holiday is likely to be to her children? Time with their mum, on their own, where they can forget about school/work/home for a few days and enjoy themselves together?

Not everything is about dsc. And that’s ok.

They’re not a nuclear family unit and OP is under no obligation to pretend they are.

Then SD owes her zero respect.