Tricked Into Fucking Sister

Your girlfriend confesses to a femdom fetish and says she want to tie you up, blindfold you, and dominate you. At first you think its hot and agree but when you're there, all tied up, your blindfold gets ripped off and suddenly your sister is standing over you. She shoved her panties in your mouth and explains that your girlfriend never had a femdom fetish...she had a brother sister fetish and she was willing to do anything to see the real thing and make it happen. Suddenly you hear your girlfriends laugh and look to your right where you notice her hands are already in her pants. Your sister tells you not to fight it, and then mounts you...