MIDA-308 I witnessed my withdrawn, virgin sister masturbating and lost my reason. I didn't notice my sister was cumming so I started pounding her like crazy.

"I saw it..." My sister's room is messy and disorganized. She's been wearing the same ugly T-shirt since middle school. She reads erotic doujinshi and has poor eyesight. To top it off, she's a shut-in and a NEET. A typical "mourning girl"... Of course, since we're brother and sister, I wasn't interested. Then I saw my sister masturbating in the most serious way! At that moment, my morals, ethics, and reason all collapsed...