Budget constraints are often real and binding for people once the bills actually come due.  Sometimes we like a fun idea when we first think of or hear about it, speak too loosely about our willingness to make it happen, and then reality sets in later and we get some “buyer’s remorse.” 

If I were you I would have a heart-to-heart talk with your sister’s friends. See which of them is able and willing to contribute a full share of the costs and listen with compassion to those who cannot.  Then, given that it was your idea, I would decide if you’re willing to cover the balance or not.  (I actually don’t think it’s unreasonable that you as a sister pay a larger portion than each of the friends.)  If you cannot afford this remainder though, I would see if you can follow the friends’ idea of decreasing your length of stay or maybe moving to a cheaper alternative.