We have large age gaps.
Adults down to babies, even now we tell the adults that we are going and if they want to come they book as well.
When they were under 18 we always asked if they wanted to come. Some times they did and sometimes they didnt as we always did age appropriate holidays for the younger ones and they sometimes weren't that interested in going.
We have done breaks just our older respective kids, all of us, our kids plus one SC. If it was a year we could only afford one holiday it would be one that we could take them all.
We have never applied the rationale of what they get, do or have in the other parents or compared it to what our joint kids have. They arent the same. Our respective SC are a part of this family.
Our joint kids aren't a part of the SC other family unit.
And as we have always explained to our joint kids, anything they do or have in their other parents is because they don't get to live in the same house with their mam and dad. Our oldest child together is even mature enough to recognise they most definitely get the short end of the straw v her and yonger sibling.
She asked me and her dad never to break up because she would absolutely hate having to go between houses all the time like her older brother and sister had to. Funny even her sibling going to disney with her dad & his family wasn't enough to swing her to thinking she was missing out not having two homes.