Incest Seduction/Tell Dawn the truth

"I was just giving Kim a back rub," you say as flatly as possible.

Dawn looks the two of you up and down for several seconds. You can see her looking at Kim's bra on the carpet. The amount of guilt hanging in the air is almost choking.

"Bullshit."

"I'm serious! Kim asked if I would give her a back rub and I said yes."

"You've been rubbing a lot more than her back!" she exclaims, pointing at your pants.

You glance down and are forced into doing a double take. Your cock is tenting your sweats almost comically. Kim puts her head in her hands. You couldn't have sprung a boner this suddenly, especially under so much pressure. If anything you can feel yourself starting to deflate. But you're now pretty sure that's why Kim had attached herself to your lap. It had felt like your dick was straining, like it wasn't even able to hit full mast, when in reality it was Kim who was holding it back for you. That is while simultaneously teasing you to no end.

You don't really want to get Kim in trouble, not that Dawn is likely to believe you anyway. But you have to do something. Dawn would not be above ratting you out to mom.

The truth didn't work. Time to lie your ass off!

You could still try to buy Dawn's silence or strike some sort of deal.