Watch them

There's something about the tone of the giggling and laughing that catches my ear. I take a step back and to the side to get mostly out of sight with a clear line to the fold-out couch. Just in time too, as Hunter pushes Kate back on the bed.

"I was worried you were just going to tickle me, Hunter," Kate says with a sexy growl. My son pushes up his sister's shirt and bra, revealing her pert C-cups to my eyes. My cock grows in my pants. Hunter leans in and sucks Kate's nipple as she reaches for his cock, unzipping his pants. Once she gets it out, she strokes it vigorously. Hunter moans into her tit.

I can't believe what I am seeing and just stand there slack-jawed as I watch my children get sexy with each other.

"I wanna be inside you, sis," Hunter says.

"You can't have my pussy. You know I'm saving my cherry for Daddy," Kate says with a moan as Hunter squeezes her boob. My jaw drops and involuntarily I become harder. I can't believe that Kate wants me to take her virginity. I almost miss the next thing she says. "Take my ass like last time." I am now hard enough the chip stone. Now I can't believe that Kate just asked to get buttfucked by her brother.

They arrange themselves on the bed, Kate on her hands and knees with Hunter and his good-sized cock behind her. He reaches between her legs and rubs her pussy to get his fingers slippery, then brings them back to oil up her rosebud. He gets another dose for his cock. Kate moans whenever he touches her. I have the best side view.

"Now go slow. I'm not quite used to having it up there."

"I wouldn't hurt you, sis. Here we go." He scoots a little closer and holds his cock as he pushes it against her sphincter. Kate squeals as the head pops in. Hunter stops moving.

"Oh, my God, Hunter! You're stretching me! It hurts a little, but it feels so good!" Like a magnetic pull, my hand has drifted to the front of my pants as I slowly begin to rub the tent in my pants. Her eyes are closed and she has a hot look on her face. She's not lying about how it feels.

My son grabs his sister's hips and starts slowly rocking back and forth, getting a little farther inside her as he goes. She's moaning constantly now.

"Fuck my ass, Hunter," she whispers, seemingly not able to speak louder with his cock in her.

"You feel so good, sis," he says, a look of pure pleasure on his face. Then he starts pushing in harder, pulling on her hips as he builds up steam. Her moaning doesn't stop. I can't help but imagine that I'm the one fucking her from behind, but she's giving me her pussy.

After about a minute, Hunter groans just as Kate squeals, as they start coming at the same time. Hunter keeps pounding her ass as they keep moaning.

Finally, after a couple of minutes, they slump on the bed and rest.