In Korea, age is a big factor. You have to respect your elders even if they're 3 years older than you! (unless you're really close)
Here are some ways to call other people when you are close or related to them Older girl/sister to another girl: 언니 un-nie Older girl/sister to a boy: 누나 noo-na Older boy/brother to a girl: 오빠 o-ppa Older boy/brother to a boy: 형 hyung Mom: 엄마 um-ma Dad: 아빠 a-ppa Grandmother: 할머니 hal-muh-ni Grandfather: 할아버지 ha-la-buh-ji Younger person/sibling: 동생 dong-saeng You don't really adress them like this. You just say their name. It's basically the direct meaning of younger sibling.
Work/School related: Senior at work/school: 선배 seon-bae This can be used to adress someone that has more experience like at school, you call a person that is in a higher grade than you, 선배. This is also used at work and in the K-Pop world. At work, it's basically your senior, someone that had worked longer than you and for K-Pop, same thing. For example BTS can call Baekhyun from EXO 선배 since Baekhyun debuted earlier than BTS. You probably heard of 후배 (hoo-bae) too right. Same as 선배 but younger or have less experience at work: 후배 hoo-bae