You asked for advice, so here's mine: in my mind, "family" means just that - mom, dad, kids. Since there are already 2 kids going (a brother and sister), there's already company for each of them.
I understand kids have a need to have friends with them, but vacation does not need to include friends - especially at additional expense to you. I also understand your need to not cook during a vacation.
In my mind, the elimination of 2 friends might just bring the cost of a cruise into your range. Have you investigated a cruise for the 4 of you vs. a land vacation with 6 people (all whom enjoy good food)? Teenagers always seem to find friends on cruises, so even if your son and daughter are in that "we hate each other" stage, they will find other friends to hang out with.
Don't mean to sound mean-spirited, but I think asking parents (you) to pay for 2 additional people (teens, nearly adults), is a bit unreasonable. Since you are on a budget, your budget should provide the family you are raising with a memorable time. It's up to the other parents to entertain their kids.
I think if you sit down and seriously look at the cost of the condo, a huge amount of food for 4 teens, the near proximity of people who are not your family and the possible unease associated with that, you might find that a cruise would work for you.
That's my story and I'm sticking to it! Good luck with this situation. I realize you want to please everyone, but your husband and kids are who you really want to please.