Show her Adam

"Your brother is safe and happy. We agreed that you'll be my slut" You bask in girl's fear. "Strip! Your new life is beginning now"

"No." She answers fearfully "You are lying. He would never... What did you did to him?" Looks like Sara is far more worried about her brother's than about her own fate.

"Are you deaf or stupid?" You say sternly "I said strip before I get angry"

"No!" She repeats in anger "I won't do anything till you show me my brother!" The girl looks way stronger than you expected from a such young lass. Sara is very scared and yet she have strength to argue with you.

"Very well" You say with a smirk "I'll show you what you want to see" you grab her hand and pull her inside the garage. Then you lead her to the outer corner to find Adam roughly facefucking Lea. He is so preoccupied with this that he doesn't even notice your approach.

"Brother!" Sara screams in a mix of disgust, surprise, anger, and fear. Adam turns his head around with his dick still in Lea's mouth and says "I'm sorry, sis"
You waste no more time and push Sara into the wall. She is so shocked that she offers no resistance as you strip her naked. She is yours to enjoy.