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.
It's nice to watch how lesbian girls have fun in bed, it's especially interesting to see what they do with a strap-on. Because they do to each other what they would want men to do to them. So there is something to learn.