My parents remarried and I suddenly got a step-sister. She's a nice girl who is honest and cheerful, and even though she's much older than me, she's very affectionate, but... she unconsciously shows me her cute, defenseless bottom every day... to be honest, she's exactly my type. But there's no way I'd ever do something like that to my family!! I've been struggling with this, and one day I've reached my limit...