Well, this is surprising.
Yesterday was yet another family event at Uncle's. I had plans to meet up with a friend in the area earlier, and called my parents on the way to ask if they were picking Grandpa up, or could I leave the car at their place and join them on the way to Uncle's. Mom said that it was up to Uncle to arrange Grandpa's transportation for that day, so sure they had room.
15 minutes later Mom calls me back very upset. Grandpa called her to ask when she was coming to pick him up. Cousin1 was supposed to pick him up, but Grandpa's logic decided that since Mom had bought him some groceries it made sense that she would bring him the groceries and drive him to Uncle's. (Mom's logic said to bring the groceries to Uncle's, and transfer to the drop-off driver's car). So Grandpa called Cousin2 to tell her to tell her brother that he wouldn't be needing a ride.

So Mom called Cousin1, who didn't answer and then Drama Llama Aunt's husband agreed to pick him up on their way. I asked Mom why she was doing all these phone calls when it clearly wasn't her responsibility, and she said that Grandpa started with the whole Yenta act "I won't come at all, you won't miss me...".
Mom then told me that she was not proud of what she was going to tell me, and to keep it between us, but that if I am asked to do the drop off to say that I already have plans with a friend. I asked if I was expected to be the return driver, and she said she didn't know, but to expect such a request. I told her not to worry, I can take care of myself.
Then Mom started talking a little about how it inconveniences her as much as it inconveniences Uncle, that he also has three kids who can drive, etc. etc. I reminded her that for SIX years I was the only one driving Grandpa, and cut off her "but you volunteered/it's on your way/everyone thought you liked doing it" politely.
Drama Llama's Aunt's husband also volunteered to drive Grandpa back since "it's on his way" while his wife made a sour face, so that headed off any further discussion on the matter.
So, things are starting to change - slowly. I'll try to help my mom out with that spine of hers in the future. It's going to be uphill work!