Helping a girl out...

You are so lost in the depravity of Riley, your baby girl, riding on your dick like a cheap hooker, that it takes Riley's gasp for you to realize....
.... you never locked the bedroom door.
"Oh... fuuuck...." Riley stops bouncing as she stares at the doorway.
Allison, your niece, is staring, open-mouthed at the taboo spectacle.
Oh no.......
"Allison.... um....." what could you possibly fucking say? You are terrified and resigned to disaster. But then, you notice... Allison has a hand shoved down the front of her shorts...
"Alli.... come here..." Riley climbs off you, running over to take her cousin by the shoulders. "Please... you can't tell anyone about this!" Allison is staring at your cock, and Riley notices.

"I won't tell anyone..." she sounds like she's in shock. "You... you said you wanted to fuck your dad, but I didn't believe you..."
"He's pretty big, Alli. It might help you.... prepare...."
"What? What are you talking about?" You sit up.
Riley looks at you wickedly. "Well, Alli and I were sharing our deepest sexual fantasies... mine was you, clearly. Alli wants to get fucked by a big black cock. I told her that she's not ready for that, she's too small, it'll hurt a lot, but..."
"You're so thick, Uncle James... Not as big as those black guys in pornos, but..."
Riley starts to tug at Alli's shirt from behind. "Maybe... you could help her.... prepare her for a bigger cock?"
Alli starts nodding. "Yes... I... I want to be ready for my first one..."
You think fast. You're already fucking your daughter. By comparison, what's fucking your niece? You don't even share any blood! If she's willing...
"If you really want that, Alli, I'd be honored to help you out..." you say it gently, keeping your burning lust at bay. You're in for a penny, might as well get in for a pound.
Alli nods, mesmerized. "I'd... I'd like that...." Riley pulls Allison's shirt over her head from behind, never breaking eye contact with you. "He'll break you in gently, Alli."
It's so strange. This quiet, sometimes bitchy teenager, who barely says two words to you, is transforming in front of your eyes. She bites her lip as Riley frees her tits from the bikini top. They are perfectly sized and perky.
Riley sees you staring. "Is... is she prettier than me, daddy?"
You reach out to take both girls by the hand. "No, baby. You're both absolutely beautiful." Riley drops to her knees, sucking your cock. Alli looks up at you, and you stroke her cheek.
"You sure you're ok with this, Alli?"
For answer, Alli pulls your head down to her face, kissing you demurely.
"Mmhmm.... I know you'll be gentle... and..." she looks down at Riley... "You have a nice dick, Uncle James."
The girls put you on your back, trading off on your cock with their young lips, back and forth. Riley climbs up to kiss your mouth.
"If you fuck her good, I think we'll be okay..." she whispers.
You nod, and look down. "Allison come here."
You put your niece on her back, kneeling down to finger and lick her young cunt. She has a small patch of red pubic hair, and her little pussy is so cute and unspoiled.
"You're not a virgin, are you Allison?" You say as you move up to lay your cock against her slit.
"No, Uncle James. I had sex with Billy, my last boyfriend, until Daddy caught us." She lays her arms back, staring up at you while Riley kisses your neck and strokes your cock, rubbing it against Allison's entrance. "He beat him up..." Allison's lip starts to tremble.. "...he gave him a black eye and then made me break up with him."
You lean down to stroke her hair. "Don't worry, Allison. If Jack tries to beat me up, I'll break his arms." Allison's laugh turns to a moan, eyes widening, as Riley pushes you inside her.
"Ooooooh..... Uncle... Jaaaaames....." Allison's eyes pop, and then squeeze shut as she writhes. "That's stretching me soooo goooood......"
You penetrate slowly. Holy flaming shitballs, this girl is tiiiiight! You are legitimately afraid of ripping her in half.
"ss....sslow....please.... slow..."
"Don't worry, Alli. I wont hurt you."
You spend the next 5 minutes or so, slowly, deliberately, gently, working your way into her visegrip cunt, until you are finally able to bottom out, to which Allison gives a long gurgling moan.
"Never... never felt anything like this before..." you start to pick up the pace as Allison mumbles. Her eyes are fluttering, and Riley starts suckling on her tits as you break her in.
For the next ten minutes, you ride Allison with careful, controlled aggression, only picking up the pace when she asks for it, and giving frequent recovery breaks, fucking Riley to a screaming orgasm while Allison rubs her own clit and watches.
With Riley panting and shaking, you return to Allison.
"Are you using birth control, sweetie?"
"Yes, sir." she nods. "I take Enpresse. Mom made sure I had them, and Daddy doesn't know."
"I'm going to cum inside you..." You pump faster and harder, and she starts to whine and moan the way only young girls can. "Is that alright...?"
"Yes! Please fill me up, Uncle James!" She grips your shoulders, grinding her hips and writhing in counterpoint to your thrusts. The motion and tightness are too much.
"Oh..OH!!" You squeeze your eyes shut as you explode inside Allison. You can feel the cum squirt out as you thrust.
"So full..... MMMMM!!!!" Allison's pussy starts shuddering, and she screams "I'm cumming tooooo!!!!!!!!!!!!!!!!"
You collapse. It's a wonder you don't look down and see cloven hooves where your feet should be.
"Thank you.... that was amazing..."
Riley climbs onto you, reaching over to touch Alli's leg. "Does it hurt?"
"Yeah...it's real sore... but it was so good.... I feel like... I'll be ready now." She rolls over to kiss your cheek.
"Well I'm glad you two are happy. The three of us are going to have a lot of fun in hell."
"Stop it, daddy."
Riley kisses your other cheek, and somehow, it makes you feel a little less like a soulless monster.
Together, the three of you clean up, change the sheets and make the bed. You grab a beer from the fridge and spill it in the bedroom, in case Jenna wonders why the sheets are fresh.
In an hour, the girls are back in Riley's room, gushing about the dirty little encounter. As you walk past, you can hear Allison talking about finding her first big black cock, and how she's ready now, and won't be afraid. You roll your eyes. It's always nice to help out.
You swallow your guilt and shame with a shot of whiskey as the headlights of an Uber car swing into the driveway.