Just how fucking loud does he jack off that his sister can hear him through the closed doors? And how the hell did he even drop the picture? He didn't have any pockets, so the only option was to carry it in his hand. Are you telling me he dropped the picture from his hands and didn't notice it?
Genuine question to the readers, because this isn't the first time I see this in manga, did you ever call out someone's name while masturbating? I'm not talking about imagining someone, but audibly calling out someone's name.
So not only did she fail to "save" her brother, she turned into a pervert too