Whatever5 I think there is some truth in the privileged argument.
Generally speaking, the more affluent a family, the longer the children are cosseted and treated as children. This makes sense if you can afford it. We now know that a human brain isn't fully developed until about 25 years old; we live in a society where the training/education needed for a "good job" or "career" takes a person well past 18 years old. If you can keep your fledglings "in the nest" a little longer, under your guidance and protection to get them as strong and ready for the world as possible.
I know that I wouldn't have 7 years of tertiary education and wouldn't have become a high earner without my dad's support in my late teens and early 20s. He made up any difference in uni costs that scholarships didn't cover; when I got my first professional job in another city he gave me first and last months rent and money to buy three suits and a formal coat. He bought me a sensible used car. From his point of view, it made sense. He was a high earner, could afford it and was committed to helping me become a successful adult. He was deeply invested in my and my sisters both emotionally and financially. And yes, he paid for a few holidays while I was at uni.
Contrast this with other kids at my comprehensive who worked 20 hours a week while in high school to help kick money into their families' budgets. I am sure their parents loved them as much, but the financial realities were very different. They simply didn't have the luxury of extending adolescence for their dc. They needed their dc to shoulder adult burdens asap.
Now, I just don't know enough about the OP to say what's going on here. She might be skint and in a situation where the family doesn't have much to give to the kids beyond the basics. (Then I wonder I why they have chosen this holiday. Cut your clothe to suit your purse, etc.) She might resent paying for a child from the first marriage, and have a stone for a heart. Most probably, she is frustrated with DSS1's behaviour, feels she has little control or say over it, and as a result just doesn't want to deal with him anymore, has been counting the days till he is "an adult," and wants to be done with it already.
If the latter is the case, she has a tough row to hoe. DSS1 is her husband's much loved child. He is going to see it differently; if he feels about his dc the way that I do about mine, he will never give up. Anyone pitting themselves against me loving my children would lose. I would grow to resent and distrust them deeply.