FNS-176 - I became the subject of my girlfriend's NEET sister's masturbation

My girlfriend's sister is a shut-in who always gives me the stink eye. I thought she was just a loser, but then I caught her masturbating. Seeing her hidden, curvy body—huge breasts, slim waist, and thick ass—under those baggy clothes turned me on instantly. Now, we're secretly hooking up behind my girlfriend's back.