Sister

After some contemplation you decide the best way to do it would be having your sister do the dirty work herself, thereby absolving you from blame.

He comes over the next day and you fill his drink with sleeping pills while your sister showers.

You leave him tied up on a chair in her bedroom with a note placed on her bed, which reads:

"Hey sis I was invited here by your boyfriend without knowing he was in a relationship, and he told me only when I heard you in the shower. He explained that he wanted us all to have a threesome and he imagined you would be less likely to say no if I was here in person. Please do with him whatever you please, as after all your the one dating him.

My condolences"

You hide yourself in her wardrobe and peek through the slats, hoping to witness her break her own mans balls.