Dorothy has borrowed her sisters car and returned it seemingly with the residue of a fast food dinner strewn on the carpet and the seats. This unofficial arrangement will not survive unless Dorothy learns to take proper care of the car, after all she is not technically old enough to drive, something which would get them both in trouble with their parents.
The compromise, reached by Alex, is that she will give Dorothy the punishment she deserves. In this case a spanking across her sisters lap followed by the hairbrush on her bare bottom. Poor Dorothy, Alex is such a tyrant!