Is this or me being stupid?

If she was using his computer, it's likely that she was sending y pictures to her boyfriend, then forgot to empty the trash. People are genetically programmed NOT to be attracted to family members (it's an issue of too-similar pheromones), since uous offspring are usually unhealthy and can carry genetic disease. It's almost impossible for a person to be truly attracted to their sibling. And his being close with his sister is a good thing; my boyfriend is best friends with his sister, and since he's used to being around her, he understands my needs better and doesn't freak out when I get all hormonal and cry over silly things. Plus she's there when he needs advice on stuff like how to cheer me up or what to buy me for Christmas. :) So please please don't mention this to your boyfriend again; he's not cheating with his sister, and to accuse him of it will scare him away!