Every argument being brought up, could certainly be countered. A medical problem with a sister and missed time together, could easily become a medical problem with her husband who really wanted family time at the Poly, but instead settled for extended family time at a resort he did not want to stay at. Disappointing a sister, as compared to disappointing her own husband. The OP already mentioned that she took a trip this year, without her husband and that she and her sisters stayed in separate resorts. There were no problems reported. If the sisters want to invite themselves into an already planned vacation, I see no reason why the OP has to change things around to accomodate them.
We don't know much about the OP, but it sounds like a fairly large age difference amongst the sisters. If they were 12 and 15 when she was born, do we really know how much sister bonding there truly was? My youngest sister was 8 when I got married and moved out of the house. I hardly had time to get to know her before I was off and starting my own family. Irregardless though, the age difference between sisters and the kids as well, can possibly add to the difficulty of trying to coordinate the groups. I'm sorry, but beyond compromising my existing and already planned vacation to include a grand gatherings event and a few scheduled meals together, I wouldn't do much more than that.
The OP clearly needs to do what's best for her own family.