We did the tour in May and loved it. Our group was me, DH, DD the CM, and The Ladies (my Mom and Aunt - 80 somethings). We all loved the tour. You go in open air buses. I agree that the roads are sometimes steep and curvy so but you sit 5 across on the seats so I'd just put you MIL in the middle and she should be fine. Don't know your MILs age but I think she'll love the tour. And you're back on St. Thomas in time for lunch and shopping.
They stop at the Anneburg ruins and give you time to look around, also some time at the Virgin Islands National Park and other stops. We had time at the end of the tour to check out the shops near the boat on St. John before returning to St. Thomas.
One warning, you have a 45 minute boat ride over to St. Johns which can be a little rough. If anyone is prone to sea sickness this is definitely the time for the Bonine or Sea Safe. The boat had an enclosed inside area and an open upper deck. Be sure to use sunscreen if you plan to sit on the top deck. If bobbidiboo
hadn't shared her sunscreen, I'd have been cooked.