Hyeong or hyung (형) and oppa (오빠) both mean "older brother." However, only males can use hyung and only females can use oppa.
The female equivalents are eonni or unni (언니) which is what a female would call her older sister. And nuna (누나) is what a male would call his older sister.
Curiously enough, there is just one word for a younger brother or sister. And that's dongsaeng (동생). And both males and females use that word.
wah. as what yisi says, the korean wave has finally hit me!
suddenly, I have the urge to take up HMK1 as my UE! Korean Language 1. (: