The derivation of this word is from "The Stranger" (sitting on one's hand until it is numb, and then pleasuring oneself). Following the same principle, the Estranger is when a brother sits on his sister's hand until it is numb, and then she pleasures him. The numbness negates any incestuous pretense.
Jessica thought her half brother was hot, but she was afraid to act on the impulse. Just so it wasn't weird, he suggested that she give him the estranger.