Why my sister walks half naked when we are home alone?

My sister is 26 years old and she leaves the bathroom door open and she walks half naked in front of me she do that since our parents travelled... she knows I get shy, and I don't feel comfortable talking to her while she is naked but she do it... at the same time she hide her body in a strange way other times... please explain what does she wants !!
Why my sister walks half naked when we are home alone?
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