It has been over 6 years since we took a "family vacation" that was just our immediate family (me, wife, kid(s)). And that vacation wasn't supposed to be just us (it was to be my wife's entire family - us, GMIL, GFIL, FIL, MIL, her 3 stepsisters, 3 cousins and 2 hangers-on), but we were forced to reschedule due to pregnancy.
Last spring, we decided that the 4 of us would take a cruise this spring (we let the kids choose Disney or cruise, they took the cruise). We got the cabin booked and were all set. A week later, my wife tells me "my parents managed to get a room on our ship, so they're going too!" They invited themselves along on our cruise. You know, the cruise that was supposed to be just the 4 of us because we've never had a proper "just us" family vacation (my folks rent a house at the shore for a week every summer and my whole side of the family goes, which is its own adventure).
About 6 months later, MIL told wife that they invited our nephews (my BIL's kids) to come too! But don't worry - they'll stay in my in-laws' cabin, and they'll be "responsible" for them. Except for the part where they have to be chased down when they break curfew, or they go on a shore excursion that's more involved than sitting on a bus and we get to chase them down in a foreign country so they don't miss the boat.
I'm sure it'll all work out fine though. Just like our last cruise where there were seven responsible adult family members and we got lots of free kid-watching. Wait...no, that's not what happened. My wife ended up sitting in our cabin every night (except one) watching our kids because no one offered to let us have a date night on board.