This ship is so stable that you can barely ever feel her move. If the seas are rough enough and in just the right wave direction, any ship will move some. But tell your sister she'll never feel much of anything.
Many people still feel a touch of seasickness, and take meclazine (bonine) and feel no symptoms.
I'm Steve Cooper. I love Royal Caribbean, and recently discovered NCL and love them too. I've also cruised Princess and HAL.
NCL Pearl Alaska Glacier Bay
RCI Liberty of the Seas E. Caribbean