hey hun, there could b a completely simple explaination to this, does you sister have long hair??
in the original post you said it happenes mainly after the shower, right?
well when you have a shower everyones knows some of your hair comes out and clogs the drain!, so it could b that your sisters hair is coming out and becasuse of its lengh it is getting caught in her bum, and staying there during her shower untill she is out and notices it
she doesnt nessesarily have to have long hair for it to happen either, hope that was some help try not to worry hun cya xoxoxo
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