I would ignore the fact that the cousins are being comped. That doesn't figure into the equation. What you shouldn't ignore is that the trip is being planned by someone who spends more money than you are comfortable with - which makes the vacation too rich for your blood, so to speak.
It's time to tell your aunt that you are so sorry, but you cannot go on this trip. It's really important for you to do this now to establish precedent. If you don't, the trips will just get more and more expensive. This year, they'll go on their own. Next year, either they won't invite you or the aunt will think of a way to make things easier or more quitable for you in order for you to be willing to go.
Look, I had to do something similar, and it was hard. Our entire extended family on my husband's side used to go to a big beach house and split the costs. But the costs really weren't split - the other kids were older and started sleeping in separate rooms while my kids slept in sleeping bags in the big closet in our room, yet we were supposed to pay the same amount of money as the others. I had to talk about it with the family and tell them that my husband and I needed to be able to take a vacation separate from the family too, but the expenses of this extended family trip were so much that it hurt our ability to have a separate vacation, and we needed to work out a difference in the cost since we were only using one room. They "got" it, and after that, the cost was split dfferently. That made it so that couples and families like mine who used only one room got charged less. If I hadn't spoken up, nothing would have changed.
So don't let this go. Accept the possibility that you won't be going again, and if things do work out to your betterment, consider yourself lucky!