A weird situation seen a freind sister naked?

ok i went to a friends house his parents were gone to work his sister was supposed to take us to the movie theater on her way to work then his parents was gonna be there to pick us up. my friend knocks on door of bathroom and says hurry up to her. she says she is. well 5 minutes later he tells her we gonna miss our showtime and for her to hurry. im standing in hall by him i was talking to him and he was telling her to hurry and about our showtime. well she opened door completely naked wearing a towel on her head topless and bottomless and says i'll be done when i get done leave me alone and ill get done faster then walks from bathroom to her bedroom. i thought it was very weird but thought she was hot so i enjoyed the little show. my sister would've used the towel to cover herself and not her head. why did his sister do this?