Ask her to join

You take out your penis and show it to sis. "Can you come help us out?" She comes over and looks at Mom's dripping pussy. "Yeah!" She jumps against mom's pussy sucking my cum from it. Mom's eyes widen. Shocked. "Ta da! Done!" We all get up and get cleaned up.

Later on sis come into my room. "Hey big brow. Can we do something new."