I've enjoyed reading all the perspectives on this thread.
For our family, we deal with some of the same issues, compounded by the fact that I have a 21 year old step daughter (that has always lived with us and been a part of our nuclear family full time), a 10 year old daughter, and a 3 year old daughter.
My 21 year old has graduated college and works full time in a professional position. She probably makes more money than I do, and has limited expenses. We're very proud of her.
But, now that I've had my youngest daughter, that bumps our party up to either 5 or 6, depending on if my Mom goes along. My mom accompanies us sometimes and chips in financially, which is nice. This March we went on a trip to WDW and we stayed at Bonnet Creek in a time share, so accommodations worked out. We rented a van, which cost more than a car, of course, but that was a fine expense. I did ask the 21 year old to buy her park admission. I don't know if that was the appropriate thing to do, as she was VERY offended and brought it up several times during the trip. But, I work an additional part time job to pay for our trips. She helps me out at the part time job and gets paid for it. It seemed fair to ask her to use that extra money to pay for her ticket, versus me having to work extra hours to buy hers while she spent her money on make up. I still don't know if it was the right thing to do.
I was pricing Disney Cruises earlier in the year. She's been on two of them with us and her middle sister before the little one was born. I was honestly not planning on including her on that one because the cabin sleeps four. Somebody mentioned something to her about it and she went crazy, just couldn't believe we'd consider going without her, but the difference in including her was over 2k, which she didn't want to pay either. I told her I was just pricing them and just let that ship sail (haha) but it seems a little unfair that the baby won't get the same experiences the oldest got because I can't afford to have another adult in the room.
My family was more traditional so my grandmother and mom always took me until I started dating my husband and had a full time job, at which time my grandmother passed away and my mom and I started going halfsies. I didn't have younger siblings, though, so I don't know how that would have played out. I'm pretty sure my grandmother brought my sister until she was 24 or so.
The fact that she's not my biological child plays a part as well, as I'm pretty sensitive to her feeling not included and don't want these kinds of decisions to add to that.
For now, my solution is to just try to find time share places that will accommodate us all. It will keep us from our favorite vacation accommodations, but it's worth it to keep everyone happy.