NTA

If they couldn't afford to send all 5 members of the family including their step-grandchild, then they could not afford the gift and it SHOULD NOT have been gifted to any of them.

16 is not a child. They are more than old enough to understand that vacations cost money, so please don't speak down to OP that way. There are so many alternatives that could have been offered. Delay the trip and save more money, let OPs mom chip in, change to a cheaper vacation destination, talk to OP in advance and discuss a future vacation, but no. They chose to exclude OP AND discuss their fun vacation in front of OP after the fact.

This has absolutely nothing to do with entitlement and everything to do exclusion. OP is NTA in any way, shape, or form.