David snuck into the room quitely, already completely naked. Nothing tipped either of them off, as his sister's girlfriend continued to lick her clit. He snuck closer, cock stiffening as he did, until finally he figure there was no going back regardless. Much to his delight, Laurie didn't even flinch when the head of his cock brushed her nose, and she just started to suck on his cock like it had been there the whole time. He heard his sister moan under her, still eating Laurie's pussy. David tensed up as Laurie opened her eyes and looked up at him with a smile wrapped tightly around the end of his cock. He gently mouthed the words "Can I cum?" silently to her as his sister continued to lick her snatch. Laurie loosened her mouth a bit and let him slide in farther, sending him almost over the edge. As he tried to stay as quiet as possible, he heard his sister ask "... what's going on down there?" from underneath. Laurie smiled and let David pop out of her mouth "Oh, it's just your brother sticking his cock in my mouth." she giggled, looking right at David the whole time, "... and now, he's going to fuck you while I watch." Laurie added, holding his sisters legs open and pinning her under her. David looked down at his sister's exposed pussy, and then at her girlfriend's evil grin, and touched the head of his cock to his sister's cunt, and leaned forward into her.