Being asked by his girlfriend to help take care of his sister and the ending

Her sister, Karen, is close to me. At first I thought it was annoying, but … My feelings changed after I started to suspect that the person she was with was a man. My uneasy feeling for her … I distracted myself with your sister … If you're going to cheat on me, I'll cheat first … That's how I feel, but … Actions towards Karen, who accepts anything, have escalated. Infidelity, betrayal, sex alone is not enough, blindfolds, handcuffs, training, etc.

Being asked by his girlfriend to help take care of his sister and the ending