Should I give in to my daughter's demands and am I not being reasonable?

So let me see if I've got this straight.

Per your daughter:

Your boyfriend is "random". But hers is "family".

Your boyfriend staying around is "bad". But hers sticking around is "good".

Your son should be bored and by himself because "reasons". And her boyfriend isn't required to step up and be nice to your son.

And being together as a family is some 24/7 thing, when the reality is that people will go off by themselves at times, or check their phones, etc. But the whole family still has to be the only ones sleeping at the rented house because "reasons".

She's a guest. You're the host. She gets to suggest things, and ask nicely if there's something she really needs. E. g. if she had gone vegetarian and you were making hamburgers every night, I'd say she'd be in her rights to ask for something different or ask if there's fridge room for her to bring something for herself or if you would mind if she brought groceries and used your kitchen and cooked something. All of that is reasonable. So would be, for example, asking for a ride to the airport or the like.

But she doesn't get to make demands, and she doesn't get to decide on the guest list. You're the hostess. YOU decide on the guest list.

You're daughter is being exceptionally unreasonable and a little silly. At age 26, she expects a "family" vacation to put everyone into concrete from when she was 14, except for her?

Life doesn't work that way.

Tell her you have to be fair to all and that everyone can stay for as long as they like, as previously arranged.

Do not entertain further discussion about your (sensible and reasonable) decision.