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.