I can relate to this OP. I have 2 DD, 21 & 19. Things aren't always fair between the two and you know what, that's life, and the sooner they learn life's not fair and are able to deal with it the better they will be for it.
If I had to pay more for DD21's college because of her losing her scholarship you dang well better believe she wouldn't be coming on vacation with us. I don't care if younger DD came on her trip, this is a different set of circumstances now. You either have the money or you don't. Don't put it on a credit card. Take younger DD and her friend and go on the trip. Explain to older DD you are paying more towards her college now and sorry the money isn't there to bring her along. And she doesn't have the money to go on the trip either, if she somehow comes in to some money it should go towards paying down her loans. Its not like this is a once in a lifetime trip, OP said the girls have been there 6 or 7 times. Our actions have consequences and sometimes it sucks, but if the money is not there, its not. It doesn't have anything to do with slighting one kid over the other, its about being financially responsible!
We went on vacation and didn't take oldest DD. She had to stay home and retake a class in summer school she didn't get a good enough grade in. I felt bad she couldn't go, but that's life. She survived fine, and got an A in the class. It was a good life lesson for her.