You love your family. Your mom has trained your sister to be available to you at all times, just like mommy is. You can fuck your sister while she does yoga or while she is on the phone with her boyfriend. Your 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 sister for head and mom makes her do it. They are sexually available to you at all times.