Incest Seduction/Push Kim onto the floor

"Eep!"

You shove Kim as she's rising sending her tumbling off of the couch to the carpet at your feet. You catch fleeting glances of her nubile bits as she rolls through the air - which only adds to your arousal - before her inevitable thump against the floor.

"Owww!"

"Huh? Kim? What are you doing?" Dawn, who had been walking along the back of the room, comes closer to investigate.

Perhaps you didn't think this through...

"Oh, uh, nothing," you reply nervously, switching your gaze between your two sisters. "She's just horsing around. She's fine."

Kim hastily pulls up on her shorts before attending to her shirt, but Dawn doesn't have very far to walk. As Dawn rounds the end of the couch, your youngest sister still has her tits out, albeit facing the floor, while her bra rests next to her on the carpet.

"What the fuck is this?"

"Zack's a jerk!" Kim angrily responds, still fighting to get her arms into the top bunched at her neck.

"Hold up a sec!"

"Yeah, no shit," Dawn replies. "But what's going on?"

Kim sits on the carpet most of the way decent and glares at you with a mix of guilt and frustration. But it looks like she's leaving the explaining up to you.

Best to tell Dawn the truth: you were giving Kim a back rub

You're not sure what to tell Dawn, but it CAN'T be the truth!

Maybe it's best you reason with her... negotiate, bribe, whatever you want to call it.