Big Brothers always make fun of little sisters, even if they're only two minutes apart.
You see Rachel Berry-Puckerman is the 'younger' sister of McKinley's badass Noah Puckerman, like all siblings, they fight. The only time they get along is when they have to go to the foster home on olive street. Outside of school Puckermans stick together, due to the terrible home life.
So being a good big brother puck looks out for his little sis. Even if it's from keeping slushies away or to having boys 100 feet away from her precious existence. Including his best friend Finn Hudson.
What happens when pucks worst fear comes true, and his best bro falls for his little sis? Heartbreak and tension rises within the three causing trust issues and disagreement between the two close siblings.