I would say something like "What would you like to do with regards to travel arrangements for your sisters baby shower?" Then, only negotiate if there is something that involves you and you don't want to or can't do it. Otherwise, just say that you understand or it's fine, that type of thing.
I will tell you that we (my D's and I) are taking our 3rd trip with H soon. In my case (at this point, I should say), it is about his being able to share something important with D's.
