What was I thinking?

I went shopping with my mother-in-law today. We went to Value Village and Costco. And we took the Twinkies of course. Somewhere in there I mentioned my fill appointment tomorrow and then invited her to ride along?? She said OK. OMGeeeeee! Nearly four hours in the car with her? What the fuck was I thinking? Don't get me wrong we get along OK for the most part but even after 14 years of knowing her she still makes me nervous. My biggest problem with her right now is I feel like she's looking for a way to start shit between the Hubster and myself. She's made a couple of comments to me recently that make him look bad. When I brought it up with him his version of events was quite different. Not just perspective different either. My marriage and my family is not a game, if this keeps up I will choose a side and it won't be hers.