Take care of the tent in my pants

I know I shouldn't have these thoughts but I start with the image of my sister's lips wrapped around the red dildo she found. My hand moves to my pants and I begin to rub my aching cock up and down through the fabric.

Then I begin picturing Becka's lips wrapped around the end of my cock instead of the dildo, with her big brown eyes staring up at me. I unzip my pants and bring my hard and soon fully lengthened cock out into the cool air of the bedroom.

My fantasy continues on to my sister sliding her lips down the length of my cock and then slowly back up leaving a glistening layer of moisture behind. I am now jerking my fist up and down my steel hard shaft. The fly of my pants is uncomfortable around the thick base of my cock. I drop my pants and boxers down around my ankles and sit on the edge of the bed. I grab hold of my now fully freed member rubbing the prodigious precum leaking from the head of my cock up and down the length of my shaft. This lubrication allows me to pick up the pace as I lay back on my bed and envision my sister now deepthroating my whole 8 inches, with her mouth stretched around the wide base of my shaft. I imagine the dildo back in Becka's hand thrusting in and out of Megan who I visualize lying next to me on the bed as my sister works us both towards orgasm. I feel my orgasm approaching and imagine Megan opening her mouth in her characteristic pre-orgasmic way and she starts to say my name.

"Dan!!!"

My eyes snap open, I look down the length of my body, past my cock, which is ready to burst and towards the door where Samantha, my sister-in-law, is standing with a look of shock on her face.