16 year old, not sure what to do when best freinds 13yo sister is trying to supposedly give signs. What do I do?

Anonymous
So basically what happened was I was dropped off at my friends house (his sister was there alone) and I went to the toilet. One of their babies knocked on the door and I shouted out "im in here", I then did the usual flish toilet wahs hands etc right in the room directly next to the living room. I then get out and see her completely naked changing facing away from me in the living room. I am thinking no way can she be that deaf, and I go into another room and yellout from there. My friend who is 17 basically also told her, after she called a classmate a creep for wanting to see a girl naked, that all boys/men want to see girls naked. My friend is learning to drive and me and her have spent some time walking, and it seems she is putting on more makup than she usually does. Now im kinda worried because I mean she's 13 and im practically 17. What do I do? Do I ask her if it was intentional? Is she trying to do something? Sorry for shitty lame time comsuming question.
16 year old, not sure what to do when best freinds 13yo sister is trying to supposedly give signs. What do I do?
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