it takes a pretty shitty mother to sell it to their own children that they aren't as loved as their siblings, just so they can hope their own child develops the bitterness towards the father that matches hers
Children draw their own conclusions, based on how they are treated and how that makes them feel. Your assumption that mum is the driving force behind any discontent as a result of her bitterness is too simplistic even if it does suit your narrative.
You're not honestly telling me you'd be able to sit and explain to your child that, sorry we can't go on holiday again this year because we can't afford to take your two teenage siblings in school holidays so we're going nowhere at all. And then your child says but my two teenage siblings went to X y and z place with their mum (because they will know if they have any interaction with the older kids) and you're gonna go yes that's right, sorry
There are many things that can happen to make a holiday accessible for all concerned. These were dismissed, however, because step mum doesn’t want the cheaper option or to take less holidays or just to think outside the box a bit. My children have spent wet weeks in caravans for years because that’s what we could afford (sod all forthcoming from the ex, that’s for sure, just so we’re clear). Did they mind? Did they have fun? Was it what we made it? Did we resolve to make the most of it? Or did we whinge and whine that dad was abroad again with his new family and you didn’t get to go because…well, he’ll have to explain that to them ‘cos it doesn’t make sense to me.
But what you're saying is its okay for the second children to be worse off but not the first kids
Not at all. This is very complex for individual families to get right. Interestingly enough, however, I don’t think I ever see these boards suggest compromise of any kind of their kids are the ones that are better off.