Hearing a noise from the bedroom, Matt softly opens the door.  In the darkness Matt can make out his children, wrapped up in a passionate embrace.  Instantly Matt realized what happened, that Kat thinking it was him, allowed Brad to make love to her.<br/><br/>Hearing their passionate moans, seeing Brad fucking his sister, smelling the sweet smell of sex, excited Matt. Watching silently, Matt accepted that he could not pass judgment, after all he was fucking Kat too.<br/><br/>Without making a sound, Matt pulled off his sweats as he quietly slid into the bed behind Kat, and plunged his cock deep into her ass as Brad continued to make love to Kat.   Matt continued fucking Kat's ass as the shock of having someone else in the bed stopped Brad and Kat's lovemaking.     But Matt continued, hoping that they would realize and understand the broken boundaries that they have just crossed.