See, Bonsoir, although I'm not disputing what you say at all - you know your situation best, I'm pretty sure that sm and ex would tell the world if they could that in 'the ultimate Disney mum who only cares about going on holiday.
The reason I say this, is because ex and sm are always complaining about what I see as really really minor health ailments that I just would not bother a doctor with. Sm is always getting dd prescribed with one cream or another for stuff I would just let go. She moans about her teeth, her hair, her weight, the quality of her shoes (but neither will buy her any), the quality of her clothing, her academic progress, her behaviour. You name it, there is a complaint to be made. All of these things are not on my register, they are nothing, they are nonsense, fabricated nonsense to make me look bad, and to highlight inadequacies in my parenting.
Now I mention the holidays because my partner and I do go on holiday a fair bit. We are lucky that we can and don't for a minute take that for granted, so in essence, dd is a well travelled little girl. We aren't ones for 'saving up for kitchens' so while our house could do with a bit of work, we prefer to spend our money while we're living. For some reason, this annoys the shit out of my ex and he referred to my 'jetset lifestyle' repeatedly in mediation. We are also a sociable family, and often have a house full of visitors at weekend and we love to eat out etc. Ex referred to this constantly as well - and the issue of the house being full especially bugged him, because he said him and sm were sick to death of dd being around a 'bunch of strangers' (ie, relatives and friends).
The point I'm making is that I could be your stepchildren's mother, because I've been described as you have described her. (ie, self-interested, lazy, party lifestyle etc).
Ex and his wife don't actually know the first thing about me. They live miles away from me, we never see each other out, don't share the same friends, our families don't speak, we don't work near each other- they literally know nothing about us. However, based merely on the fact that we go away most half terms, this is enough for them to conclude that holidays are the only thing I am interested in and everything else about my dd MUST be taken care of by them, or she will suffer.
Honestly, my dd is merely 6 years old and just could not communicate enough about our lives to them for them to have formulated the opinion of us that they do- I'm just glad that it brushes off me now and I see it for what it is- jealousy.