I'd like to bring another perspective into it I have not yet seen mentioned: the definition of family.

You MIL views the family as her and her husband, with their two kids and the (loved) hanger-on (you). Your frame of reference is you and your wife on a joint trip with the in-laws.

This is an issue of reframing. You need to explain to the MIL at some point that you and your wife are your own stand-alone family. Maybe she will grasp that herself if you decide to have kids of your own, maybe not. But right now she does not seem to get it.

So while this is a family vacation, she needs to realize it's not 'her family and you' but 'her family, your family and the SIL'. This should also make it easier to explain your 1-day off with 'your family special treat' (no intrusion please).

To compare this to a personal anecdote. It took some time for my grandparents to realize, that I am my own family. For my parents this was clear from the get go and they had my back. The scenario was simple, they[grandparents] invited my parents to some events 'children included'. Well, the child was back then in the mid twenties, finished university, living on their own for years, etc. So the re-framing was simple from my parents side. Ask your grandchild yourself, he is his own family. We still love to think about him as our little kid, but matter of fact he is long grown up, has his own responsibilities, is working towards his own family (in the sense of gf/wife/etc), we can and will not try to dictate him for some event and will not play the middle-man. Please realize that the child you had on your lap over twenty years ago is now their own fully independent person with a family of their own.

Just my 2 cents.