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Subtlety sit next to her when it's just you and her around and shove your hand down her pants or up the dress and start rubbing her
otherwise you just die alone as some disgusting creep who eats shit and steals nickers ewww wtf
a normal person way of being attracted to your sister would be brushing her hair when she is watching tv or helping her buy clothes and comfort her and give hugs And compliments.. That is the "normal" verson of that weird fetish
the less normal is what I said above psychically pleasuring her
stealing panties and licking shit and piss...you need to go to a therapist about that because that's nowhere near normal