It’s weird because I’d say it’s the opposite, there is a cohort of people who are desperate to keep step-mothers in their place by insisting that they have to sublimate their entire selves and their other children for their step-children.
Well that’s not me. I know being a step parent isn’t always an easy role, especially when they have their own bio children too with the step children’s father.
I’ve said I don’t think OP is unreasonable. But there’s zero sympathy or understanding of the SDs feelings here. She may already feel not part of things because she doesn’t see her dad as much as her half siblings, now her half siblings are off on holiday. And to Disney as well which has the ‘wow’ factor over a holiday to Cornwall for many people. OP very much comes across as not caring a shit for her SDs feelings. It’s all about whether it’s reasonable or not, forgetting there’s a child’s feelings here. And the WhatsApp from SDs mother has gone from ‘asking her to consider taking SD’ to a ‘passive aggressive message’.
Whether you like it or not, some step mums on here do seem to revel a little in the things they can do which aren’t necessarily unreasonable but leave step children feeling a bit upset.
Her dad needs to step up and do nice things with her, and with her and her half siblings to make her feel more part of things. How often does he actually see her, I really hope he isn’t every other weekend type dad.
Although it’s hard being a step parent, its also hard being a child who has to split their time between parents and houses and see half/step siblings doing things that you’re not. That doesn’t mean they should or can be included in everything, but a little understanding of feelings involved would be good. It’s not about winning an AIBU debate on mumsnet, this is a real child’s feelings and childhood which will shape the person they become. It’s also about the relationship between half siblings which you should want to be as good as possible.
Instead if avoiding talking about the holiday, talk to the child about it. Explain why she can’t go. As with everything, communication is key and making children feel reassured is important. Avoiding talking about it will just mean she’s wonders and feels like an outsider.