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.
Here the girl was hardy, so if three more men came, she would cope with them too. And in general the girl is pretty and experienced, in bed she does such things that many have never dreamed of.