How do I tell young relative her shorts are too short?

I am renting a room from my uncle and aunt. That's working out, but one thing bugs me. Their daughter, who's 15, walks around the house in nylon athletic shorts that are way too short, with her cheeks visible, and too tight as well. She'll bend over right next to me, or dance in the living room to something on the Alexa.

I feel I ought to tell her she needs to dress more modestly. On the other hand, I don't want it to seem like I'm looking at my much your second cousin that way. How should I handle it?

How do I tell young relative her shorts are too short?
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