George and Fred are twin Weasley brothers as they are running their joke shop together

Their younger brother Ron approaches

Ron: how much for this?

George & Fred (as a union): five gallons

Ron: how much for me?

George & Fred (as a union again): five gallons

Ron: but I'm your brother

George & Fred (as a union): then ten gallons

[ Ron scoffs then leaves the jokes shop with Hermione and Harry behind him ]