Karen and Melanie Series, Book 4: The Wedding by Kathleen Paul (and Kathleen Paul's Writing Adventure Club)

Karen's mom has fallen in love with a veterinarian, and the wedding is scheduled for the end of summer. Instead of helping with the planning, Karen will be busy babysitting with her best friend Melanie Honeywell. Priscilla, a six-year-old better known as Pistol, broke both her arms in a skateboard accident, and her grandmother just can't cope anymore. It's up to Karen and Melanie to keep Pistol occupied and out of trouble all summer long.

Out of the blue, a letter from Karen's father arrives, announcing the plans he's made to take Karen and his current girlfriend camping in Yellowstone. With a string of broken promises behind him, he's provided no date for the vacation nor asked whether his daughter is even interested in going. If this is like all the times before, Karen can be certain he'll either not show up at all...or arrive on her mother's wedding day!