The sister is not shy with her brother - he has known and used her body for a long time. She often bailed him out while he didn't have a steady girlfriend. Now he has a girlfriend, but he enjoys pleasing his little sister. And he always comes only in her mouth - she loves the taste of sperm.
For a certain fee now any stranger is ready to take off her clothes, spread her legs and suck the first man she meets. Every cute girl is weak in the face when she sees bucks in front of her. I wouldn't want to be a pickup artist because it's a risky business to fuck unknown holes. Sure you can use a condom, but rubber doesn't always save the day.