Gravol can be used for a few different things. It's main use is for upset stomachs although it relaxes and calms a lot of people. I am one of those who relax and get sleepy if I take it. There a lot of people that take it if they are flying somewhere, and many who wll give their kids Gravol if they are flying or going on long trips in the car. Adults take pill form, but there is liquid form for kids. If you go to the pharmacy, they should be able to help you find it. Here it is behind the counter, but you don't need a prescription for it.
This will be the first time for Justin to fly, and us having 2 kids to take, so it could be fun, but I already have the Gravol packed in the travel bag!
Maybe find out from your FSIL what it is exactly that she is upset about. Then you can let her know about the Gravol and she may calm down more. Is he a good traveller in the car?