The Annoying Younger Sibling is often used by writers as a balance to the "coolness" of the main protagonist. Typically the opposite sex of the main character.
Little brothers tend to be: dirty, good at sports, noisy, foul-mouthed, always get in trouble with authority, perverted. Usually a Bratty Half-Pint.
Younger sisters are usually: whiny, cry to get what they want, too curious, go headlong [...] [tvtropes.org]