Felix is your long time crush, you trusted your sister so much that you told her about your feelings towards Felix

After you told her how much you liked felix she invited him for dinner for you to 'get to know him' more

As the three of you had dinner you couldnt help but notice how touchy and clingy your sister is towards Felix, but you shrug it off and went to the kitchen to grab a knife but when you came back you saw your sister and Felix kissing