As has been demonstrated over and over in the replies to this thread there's no 'right' and 'wrong' answer to this issue as clearly individual circumstances can vary hugely, and thus affect what is 'fair'. What works and is fair for one family may not be so for another - it all depends.
From my own personal experience - and this is a particular bugbear of mine - it is our (mine AND DP's) youngest child who's missed out most so far as holidays are concerned. They have watched - and heard (because my stepkids tend to boast - right down to how much has been spent etc) - their older siblings describe their holidays (plural) every year .... that's a winter holiday, plus a summer holiday abroad, plus residential school trips (also usually abroad) and we have had to 'manage' that by falling back on the old cliché 'all families are different' most parents use when their child points out so and so at school has/does something they'd like. I have no qualms about that per se - but it's a different ball game when you are talking about a child's own siblings, the ones we are forever reiterating as 'family' .... though it often feels as if they are 'family' for the good bits, and not for the bad/boring bits ...... and it sticks in my craw to trot out the same old platitudes when we know (but obviously don't tell our youngest) that our inability to have holidays (and do lots of other things) has been severely impacted by the unfair (and sometimes dishonest) actions of DP's ex, and, it has to be said, the older 'children' themselves in recent years.
I'm not being very explicit - sorry - because the detail could out me but what I mean is that we have all but effectively been forced into unfair and avoidable expense over several years which has taken away our ability to have any sort of holiday at all. In recent years, my teenage stepkids (who are now adults) have contributed to that - quite significantly - by being selfish, unco-operative, stubborn (can't really elaborate on a public forum) and, in order to maintain contact with them DP has felt he's 'needed' to spend money to either 'win them round' or 'maintain' contact. My view is rather more down to earth and IMO it's more like manipulation and emotional blackmail but there you are.
Fact is - our youngest has had one holiday in a decade - one they were too young to remember 8 years ago. And this year, we might, at long bloody last, just about be able to afford one - though this could well be a 'one off', so doubly important IMO. Of course it's natural that DP would also like to bring his older 'kids' (adults) but that would all but double the cost and that'd be impossible. The alternative is to downgrade the location and length of holiday to include everyone - which, quite frankly, would then become a short break rather than a holiday and one where we'd be held ransom to the vagueries of UK weather. Personally, given OUR background I don't think that'd be fair. Why is it okay for our youngest child to miss out on the sort of opportunity and experience - i.e. going abroad, seeing a little bit of another culture, enjoying good weather - which their older siblings have enjoyed dozens of times ..... so that they - who are now young adults - can participate in what they'd probably consider a very boring UK shorty break ? I can easily see our child reaching 18, looking back, and having NO memories of a holiday AT ALL with either parent - yet the older ones can now look back upon many interesting and exciting experiences with their mother, as well as some holidays with their dad when they were younger.
No doubt some on this thread would feel it's not about the type or length of holiday blah blah - it's about 'quality time' with their dad, but again, this is where our particular family dynamics come into play and I can categorically state that my stepkids have rejected many many opportunities to spend quality time with their dad and his wider family - they are always included in special family occasions, and regular contact has always been encouraged - because they seem very reluctant to join in with anything unless there's a material gain and/or our plans include something of particular interest to them. Excuses will be made e.g. about an older relative's significant birthday because it's not very exciting, and I can't count the number of times they've pulled out of arrangements to see their dad because something better's come along.
That said ..... given their ages (young adults), given our lack of disposable income (which has taken a huge hit due to actions of ex and 'children'), given the lack of holidays our youngest has had, and the prevailing attitude towards spending time with DP generally, I will stick my neck on the line, declare myself a wicked stepmother and say I don't feel they deserve to come away even if we could afford it. It sits very badly with me that they only even make an effort for 'good' stuff - and I strongly suspect, given their lack of enthusiasm for seeing their dad, that if we do manage to go away, any objections from them won't be because they'll be sad at not spending time with him, but at the money they perceive as being spent on their youngest sibling ..... despite the fact, as I said before, we have spent much more on them in the past few years (over and above maintenance of course, and usual expenses when we see them) .... and despite the fact that if they stopped to think about it, their own actions have directly and adversely affected our youngest ..... and despite the fact our youngest will have just a small taste of what they've enjoyed for many years - and the fact they're adults!
This is a difficult situation - because when all's said and done, they are all DP's 'kids' and in an ideal world, we'd have enough money to satisfy everyone. (Then again, in an ideal world, DP's ex and older kids would have been far more considerate, honest, fair and prepared to compromise sometimes ....) But this is where we find ourselves, this is what we have to deal with, and I think what we hope to do is fair. My DP getting an attack of the 'guilts' will no doubt arise again - and I will have to deal with that very tactfully and sensitively ..... I think he knows damn well that given the overall picture, we're not 'wrong' as such, but even now, even though they're adults, he's still petrified of so-called contact being denied him if they get the hump about anything and consequently, this colours so much of their thinking. If our background had been normal, I'm sure this wouldn't be such an issue and he'd have no qualms about explaining rationally to them that for the first time in forever we were planning to give our youngest a little of what they've always had.
And FWIW, I am also the mother of a (now also adult) stepchild - whose father sometimes went away with his new wife and younger children, and sometimes also took our child. I didn't have an issue with this - and nor did our child - because, as other posters have said all sorts of additional factors can come into play such as time off school and work, suitability of holiday where there are large age gaps and so on. My ex and I had a good 'working relationship' so far as our child was concerned - never used them as a weapon, or shit-stirred - so our child felt confident in their father's love and therefore readily accepted that they couldn't always do everything with their dad all the time. They still have a great relationship now and I don't think there was ever any suggestion they felt neglected and/or rejected.
Disclaimer : these are MY circumstances, and my opinion wouldn't necessarily apply to others, but I do get very fed up at being 'told' - even impersonally - by people who (obviously) don't know me how I 'should' arrange my holiday (and god knows what else). The same applies to the OP ..... she is clearly in the best position to understand her own particular family dynamics, all its nuances, all its history, and so on. Again, in our case, it's absolutely fair to say that we have bent over backwards for well over a decade to include DP's older kids as much as we possibly could in everything - in the face of huge hostility and stress ...... I kind of feel, now they're young adults, and given the background, that it's about bloody time someone else got a 'look-in', even if only for a one-off couple of weeks. There's always this concern for the non-res kids, and rarely a thought given (except by those living in similar circs) to a resident child who also 'misses out' in various ways ..... and, finally (after this long diatribe), I personally don't think that having two parents 'together' makes up for every last 'injustice' or difficulty, it just doesn't. And it's definitely NOT our youngest's fault that DP's first marriage broke down FFS!