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.
From sex with her father, the girl got what she wanted. You can see his power, his desire to get off her arousal, his knowledge of her sensuality.