Anonymous wrote:"What would you do? "
Learn how to be a better writer.
Rude, stupid comment, PP. Do you really have nothing better to do than troll every thread and call everyone a bad writer? You do this all the time and it is so boring. I found OP's post really entertaining and well-written, and I'm an editor, which I suspect you are not if you found her post badly written.
OP: It sounds to me like your MIL just doesn't like to commit and wants to keep on winging her visits at her convenience. Can you try making it clear that while she's is welcome at your house, you would like and expect prior notice for her visits? To enforce this, from now on, when she shows up in your driveway, just text back: "Oh, I'm so sorry, we're just heading out the door for a playdate/dentist appointment/whatever. But we'd love to see you, so let's make an advance plan for later this week. What day and time work for you?"