I can't come out! I was hiding under my sister's bed and encountered a super lewd look that I normally wouldn't see!! I hid under the bed to surprise her, but I usually...

I was hiding under the bed to surprise my sister with a small prank...and then I saw her changing clothes, masturbating, having sex with her boyfriend, and seeing her sister, who I wouldn't normally see! When I think of something erotic happening right over my head, I get so erection that I can't come out! My sister found out when I was peering at my dick hard! I thought I'd get scolded, but when I saw my erect cock, my sister got turned on fire, and I was pushed down on the bed, causing it to turn into forbidden incest!