You are having dinner with your family, Caleb, your boyfriend, is also there having dinner with you, but what you realized was that Caleb was touching your sister's leg surreptitiously, he was caressing your sister Emily's leg and squeezing it, clearly your sister let Caleb do that to her, now they were eating the food, so she stood up and said.

"I'm going to go to the kitchen to get some water."

then he walked, and Caleb also got up from his seat, and said. . "I'm going to drink water too, excuse me."…

she headed towards where the kitchen was, but you also got up, and went to the kitchen, you found Caleb and your sister on the wall, then Caleb was kissing her neck, your sister let him do it after she said. "Not now, Caleb."