It's fine that my sister is wearing a zip-up hoodie without a bra, but no matter how you look at it, her chest is too open and I can't help but stare at her when she leans forward defenselessly, and I get a full erection! I can't stand it anymore, so I pull down my zipper while she's sleeping. I thought she'd find out and get angry, but she actually noticed my erection and was secretly aroused, so she pulls down the zipper and says, "I'm so curious about it"... Is this a provocation? When I realize it, she has a hand on my chest...