I hear the doorbell and you flinch: it's already time. You open your mouth, parting your soft, pretty lips and I slip the gag in. Your grey eyes are wide with anticipation, following my hands as I adjust your neck strap and tighten the harness that fixes you upright in the chair. I check the handcuffs and then drop to your feet to check the legcuffs, making sure your limbs are secured.
"Back soon."
I let her in and she glances at you as she takes in the room, before her eyes lock on me. She stands there for a moment, your mirror-image, and I catch sight of you over her shoulder staring at us, helpless in your prettiest lingerie. You look so beautiful, and as I reach in to hug her and bury my face into her long blonde hair I'm smiling at you. You're going to see everything.
I undress and lead her to our bed. She slips off her dress and she's naked underneath. I lay her down and begin to explore her body, so strange and yet so intimately familiar. We entwine and I slip into her, her eyes on me, but my eyes never leave you, gagged in the chair. She ignores you, going on top, riding my cock until I'm on the edge, then we roll over and she spreads herself wide. I thrust into her eager pussy as her nails rake my back. I see the desire in your eyes as you watch me bring her to climax, and I don't hold back as I pound into her until she cries out, wrapping her body around me, back arching in bliss.
I climax. After, I pull out of her slowly, showing you the cum thread hanging from my tip into her. Your eyes are locked on it, on my juice-slicked cock as I get off the bed and come over to you, slipping the gag from your mouth and kissing your needy lips while she dresses.
You know I never needed this from you, but you said you do.
She kisses you on the forehead before she leaves, and I lead her out. At the door, she makes me promise you'll attend your mother's barbeque on Saturday then departs back to her unsuspecting husband, no longer the harlot but now your older sister by five minutes, bonded by an oath with you.