Looking for a diaper story

Its about this girl with a twin sister, the sister has some mental issues and has to wear diapers 24/7. The family goes on a road trip where the twin sister has to wear diapers and sit in a locking car seat, but will fuss a ton because she isn't matching with her sister, the main character. The main character then decides to take one for the team and says that she will wear a diaper on the trip to her mom. Then her mom forces her into the same type of car seat where she is forced to sit with a pacifier gag in just like her sister for the few hours of the car ride until they get to a rest stop, where her and her sister are both changed in plain view of strangers.
(some of those details might be wrong, just saying it as I remember it)