Was the smell of your ass lingering in the room by the time she returned? Does your sister frequently "borrow" your t-shirts so looking in her room was a legit excuse?
When your sister and you are alone in the house, strip completely naked and put on one of your t-shirts that is long enough to cover your cock when limp. Walk into your sister's room while she is there and complain to her that she wore the shirt you're currently wearing and it still has her "scent" on it. Tell her she needs to find out what "borrowed" items smell like when they are returned. Immediately go to her dresser drawer where she keeps her panties a grab a thong. Turn your back to your sister and bend over to put her thong on right there. Your t-shirt should rise enough to expose your bare ass to her. When you have the thong pulled all the way up, turn toward your sister and take of your t-shirt. Throw it at your sister saying "Even trade! Next time you want to 'borrow' any of my t-shirts, I expect one of your "items" as a deposit!"