They do what they're told

Sally casts another confused and disappointed look at her sister, then sits on the edge of the bed, covering her breasts with her arms. Grace looks at you, biting her lip, then glances at her sister and ducks her head, blushing in shame or embarrassment before slowly sinking to her knees in front of you.

"You'll keep Sally out of it?" she murmurs as she reaches out and starts to unbuckle your belt.

"Just do your job," you growl softly. "Do it well and everything will be cool."

She presses her lips between her teeth, worry evident in her face as she unzips your pants, then reaches in to squeeze your cock through your boxers.

"I'll be good," she whispers nervously. "I'll be very good."

You look down at her as she slowly pulls your cock free, staring up past it's stiffening length at your hungry face. She takes your shaft in her slim fingers, holding it as she laps gently at the bulbous head. She kisses it, then rubs it against her cheeks, all the while staring up at you with wanton submission.

Your cock swells and throbs as she plays and teases, loving the satin feel of her skin and the warmth of her lips and tongue. When she finally sucks the tip into her mouth and swirls her hot tongue around the sensitive flesh you moan and shudder with pleasure and reach out to grab a handful of her hair.

You look over at Sally to see her face grimacing in disgust, obviously repelled at her sister's behaviour. The sight excites you more, and you sneer at her as you slowly start to rock your hips, pumping your cock a little further into Grace's mouth.

Grace drools on your cock, using her saliva as lube for her stroking hand. Her other hand reaches behind you to clutch at your thigh as she slowly bobs her head, taking you deeper with each movement.

"You're a good little cocksucker Grace," you groan. "You've had lots of practice, haven't you?"

"Mmmhmmm," she murmurs agreement around your meat, pressing herself deeper onto your swollen shaft. Her spit lubed hand slides down your cock to fondle your balls for a moment then slides back up as she pulls her mouth back, her fingers teasing over your sensitive head before she lowers her head down on you again.

This time she bobs her head quickly, slurping and gobbling as she devours you, her eyes looking up at you as she strives to bring you off. You feel your cock throbbing and pulsing, nearing your climax as you look down at the sweet redhead sucking your cock and her disgusted sister watching the performance.

You're nearing the point of no return, grimacing as you try to maintain control, loving the brute power of using this girl and making her sister watch, trying to decide where and how you're going to empty your cock.