Sam is masturbating

I stop suddenly because I can hear a liquid sound coming from the bed followed by a soft moan. Sam lays naked on the sheet, fingers rubbing her clit as her other hand gropes her substantial tit and nipple.

"Oh, Dan. Yeah, that feels good, Dan."

I'm startled and look closer, but my sister-in-law's eyes are closed. She's fantasizing about me!

"Right there! Oh, fuck me, Dan! Unh!" Her purring voice is erotic and deep, and my cock is a steel bar in my shorts. "Oh, yeah, so deep. So deep inside me, Dan. You're gonna make me come so hard on your cock, Dan."

I pull out my johnson and start rapidly jerking off. I reach back and push the door closed, but the latch makes a loud click. Sam's head quickly turns my way, and she gasps at the sight of my hard shaft in my hand.

"Oh, Dan! You know my secret now! Come here and take me. Use me. I'll buy your silence with my body," she squeezes both breasts and arches her back, moaning. "Fill my holes, Dan! I want you!" she whimpers.