His mouth drops open at the sight. On the couch Karen lays back naked, her big tits bouncing as her best friend Katie fervently grinds her pussy against his sister's thigh. Katie's thick ass and thighs bounce with every thrust and, even though he can't see it, her arm moves in a way that he knows she's fingering Karen just as emphatically as she is grinding against her.
Frozen in place, he stares, unsure how to react to the scene until Karen's face turns and sees him. Her eyes grow wide with fright as she reaches up to push Katie off of her.
"Oh my god! Katie stop. Oh my god. Brett, what are you doing home? OW!"
Katie's legs clamp down around Karen's thigh, shaking and arching her back, as an orgasm washes over her. "Fuuuuuuuuuck...", she moans out as her whole body shakes, cumming harder than Brett has ever seen a woman do before.
Both Brett and Karen can only watch as she rides out the orgasm until her body falls limply against his sister's chest. With a flushed face, she sheepishly apologizes, "Sorry, I couldn't stop it."
Pushing her off and onto the couch Karen stands up, trying to cover both her tits and her crotch. It's a futile effort, her breasts spill up and over the arm crossed over them. "Brett, you saw nothing. Do you understand? We were just... we were... Shit."
Brett already knows he'll never forget what just happened, or the unexpected reaction he's feeling. He wishes he'd seen more, seen his sister cum just as hard and uncontrollably, even wishing that he'd been able to join in. "Yeah, I don't think I do."
"What do you mean? You don't understand?" She stands there, looking back and forth, confused at the unexpected reply. "Brett, I need you to forget about this."
Finally starting to come around Katie chimes in, "Yeah, I doubt that's going to happen." She starts making a jerking motion with her hand, "You know what he's going to do tonight thinking about what he just saw."
"You've got to be kidding me!" Karen's exasperated statement comes as she turns and begins to walk away.
Katie smirks, catching him staring at his sister's swaying ass as she walks up the stairs. "It's pretty amazing isn't it?"