Personally I think opinions are being polarised on here because of the type of holiday it is, and because of how OP has failed to take into account her DSD feelings, plus people always have quite strong feelings on SC relationships on here.
I think OP is well within her rights to take her kids away with her mum, but she hasn’t considered that her step child may feel left out that all the other children are being taken to Disney (I doubt this thread would have been started if they were going to Spain as it’s not about the actual holiday, they’ve been holidaying together and apart for years and that’s never been a problem before, it’s about the fact it’s specifically a children’s holiday to Disney).
She’s saying that DSD is ‘not her child, so it’s not her problem’, which IS true, you can’t argue that, but OP doesn’t seem to recognise that that attitude can be quite hurtful when DSD has been a part of her life for at least 10 years, and the fact that they kept the holiday (which has been booked for 3-4 months) a secret from her isn’t nice either IMO, the other kids have also been put in an awkward position and one of them accidentally spilled the beans.. then instead of explaining WHY the holiday has been booked and setup this way, so there’s no confusion/hard feelings she’s quickly changed the subject on her, so she’s understandably gone home to talk to her mum about it? and her mum has asked them to consider taking her? It’s hardly surprising that that has happened.
I think the OP is right to do the holiday her way, with her mum, there’s nothing wrong with that.
IMO what is wrong is to not even think about taking her DSD in the first place (OP says she didn’t even consider it, but that IS her prerogative), and to also not consider how that could potentially make DSD feel when she found out she was the only child not going.
IMO if OP really though she was morally right, there’s not reason why she wouldn’t have been upfront with DSD, I suspect she was going to tell her after the fact, or potentially not at all.