You shouldn't hook up with your sibling's friends.
Jack is home from college for the summer vacation.
Jack wants to have a fun summer.
Jack takes his sister and her friends on fun outings throughout the summer.
Jack starts going out with his sisters best friend then breaks up with her when he heads back to college.
Jack gets to grow closer to his sister and make friends with her friends.
Jack and his sister have a fight over his treatment of her friend and their relationship suffers.