After the second stop with strangers, I would have interjected - "hey cuz, i've got the directions right here. i know where we're going, and we're gonna keep moving so we don't miss the reservation, but if you'd like to take the scenic route - well, this is a vacation, right? Here's your copy of the map. See you at the restaurant. Enjoy the walk!"

I'd go with the "okay, you pick the restaurant the concierge recommended" the first time. The second time I'd just say that I really wanted to go to the restaurant that I'd read about, and that's where I was going, and she was welcome to join me but I'd be happy to go alone.

I had a coworker like this that I occasionally had to travel with. She managed to find us the one lousy restaurant in New Orleans and then tried to order a Sex on the Beach.