The ONLY thing that would be concerning is if the SIL came up with ways to stop the daughter from traveling in future, because it seems she does like to travel. The danger is that once they are married, he may well pressure her to no longer take free travel gifts (or any gifts) from her parents, because he may resent the implication that he can't support his own family. I'd say they really need to talk that out and get an agreement that any decisions about gifts from her family to herself or their children are hers to make.
IME, people have a tendency to fall into one of two categories, generally. There are people who most value experiences, and there are people who most value things. Many, many people who grew up poor will fall into the latter category, because of their childhood deprivation. Very often, people who value things will wistfully say that they would like to travel someday, but mostly they don't mean it, because that little voice in the back of their minds will always insist that things can be used over and over again, while experiences are gone immediately, and are therefore a frivolous waste of money. If they have been raised with the whole "poor, but proud" mentality, they won't take experiences as gifts, either, because it's a point of pride that they don't waste money, even other people's money.
Now, as to the OP, he has said that he does not wish to go on this trip, so she needs to drop it. Beyond that, the future is her daughter's call.