You love your family. Your step-mom has trained your step-sister to be available to you at all times, just like step-mommy is. You can fuck your step-sister while she does yoga or while she is on the phone with her boyfriend. Your step-mom has to get work done. That doesn't mean she can't bend over her desk and let you relieve yourself while she's working. Even at family meetings at the dining table, you can ask your step-sister for head and step-mom makes her do it. They are sexually available to you at all times.