I too am prone to motion sickness - cars, planes, and ships. My physician prescribed Bonine (Meclazine) and it works for me. We also book a stateroom mid-ship. We encountered 14 foot swells the first day at sea on our Alaskan cruise last September. I didn't feel the least bit queasy, but my husband did. I took Bonine; he didn't. Have your sister-in-law check with her doctor about what would work best for her and enjoy the cruise.