Introduction. 

"I have a younger brother" = 난 동생이 "nan dongsaeng il" 

"I have a younger sister" = 내가 여동생이 "naega yeodongsaeng il"

"I have an older brother" = 난 오빠가 "nan oppa ga"

"I have an older sister" = 난 누나가 "Nan Nuna Ga"

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cr: google.com for the hangul. 

Note: Its really difficult to remember the hangul words. I myself don't learn much either. But Hangul is a lot more easier than Chinese characters. 

Thanks PhAGet3v3rl3 for this poster! :)