either way, this is a family holiday for the OP with her family, I disagree that a SP has to treat a child as theres, and my DC1 has a stepdad.
Fact is DC1 also has a dad who treats DC1 to things, its not for my DH to make DC1 his total resposibility.
DC1 does not need a 3rd parent, the 2 he already has are mroe than adequate, me and exh.
My DH is very good to DC1 dont get me wrong, and in every way he drives DC1 around, cares for DC1 and does all the things a parent would do, DH also loves DC1, but DH is not DC1s father.
In this case, and in mine, DC has another parent who loves them and treats them.
In cases like this, where the DC1 is being treated as an equal child in most ways - the DC from the first relationship end up with double than the DC from the second, and I see this in my own DC1.
Financially, DC1 is way better off than my other 2, has more money spent on them than my other 2, and wouldnt begrduge their little siblings a holiday, if I said to DC1, we are going here but we really cant afford to take you, because you are classed as a third adult, it means a second room would you mind not coming - DC1 would be fine.
If I suggested to DC1 that we canned our holiday and took all of them away for a weekends camping, instead of a few weeks in the sun, DC1 would laugh and tell me not to bother, no decent child is going to expect their parents to make that choice.
I understand the MASSIVE cost implications the OP is facing, because we have had to face them.
Thankfully my oldest isnt selfish, and knows we are doing things on a budget which means sometimes, there are difficult choices.