We’ve had family members/friends invite themselves onto our trips before. Normally for us in the end they don’t get to go but occasionally they do. We always are just honest with them that they can come if they want to but we will not change any plans to fit their needs. Even if those plans are we want to go ride pirates right now and they want to go ride splash mountain we go “ok have fun at splash mountain we will meet you at The castle when we are done”. We will also sometimes say we will spend so many days with them but other days really are “just us” days and we let them know in advance what days those are. If that’s the route we go we do take into consideration what they want to do as well to a point. If we’re at Disneyland and they want to go spend the day at a beach - sorry but no - but if they want to ride splash mountain at a certain time or go eat at a place we didn’t have listed for the day we will go along with them.