We have elder DS and younger DTwins.
We took younger DTwins to Butlins for a week. DS was unable to come, because he was at school, and stayed with nan and grandad for the week and had a great time. Also, the age gap meant that the week would have been rubbish for DTwins waiting around whilst DS did stuff they couldn't do, and vice versa.
Eldest DS did plenty with us before DTwins were born. When more siblings arrive, time is shared. DTwins don't do half the stuff DS did before them. Now, we do things all together, and we do things separately. No one gets snarky about that, because it's normal. DS wasn't phased at all about not coming to Butlins because we made sure to explain that he wouldn't be missing out on much of interest to him. It's only an issue if you make it one.
Every child doesn't have to come to everything.
DS is going away with just DH for a few days to France later this month. And DTwins aren't invited. Which is fine! Why on earth would I be telling DH he was "disgusting" for not including all our DC?
According to this thread, DTwins shouldn't ever have enjoyable week to themselves either, because their brother wasn't there.
I don't understand why in a blended family, people think their child has to come to everything, when in a nuclear family, this doesn't happen all the time.