You had confessed one of your most perverse fantasies to your girlfriend, Carmen - you wanted to fuck her younger step-daughter. As disturbing as the idea was, she was aroused by it, too, and thought it'd spice up your sex life. So, she called her into the room.
"We have a fun, little activity to do," she told her, "it's kind of like Simon Says, but we'll call it Step-Mommy Says."