I suggest a plastic drop-cloth for your sister's seat...
I have the same rule in my cars but my fiance insists on food/drinks on long trips, so I made her buy a plastic sheet to protect from her messes...and it has saved me from a few half-full sodas and a piece of chocolate cake from Jack-in-the-box.
I have the same rule in my cars but my fiance insists on food/drinks on long trips, so I made her buy a plastic sheet to protect from her messes...and it has saved me from a few half-full sodas and a piece of chocolate cake from Jack-in-the-box.