Not quite a twitter

My friend has two children - girl/woman (18 - at what point do you call them women??) and boy (15). Again, great kids. One day the boy had just come home from a lacrosse game and gave his sister a hug. All of a sudden she runs down the stairs and says 'that's creepy'. The boy looks confused for a second, then looks down, and yells downstairs 'I have a cup on!'.