Their first use is towards birth siblings. So, if you are female and have an older brother, you’d address him as 오빠, rather than his name, and if you had an older sister, you’d call her 언니. If you’re male, you’d address your older brother as 형 and your older sister as 누나. That part is simple.
However, kinship terms carry a lot of hidden meanings and over the years the terms have been expanded to being used towards people whom you have a close relationship with too. For example, a female may call her older cousin 오빠, her older best friend 오빠 and even her boyfriend 오빠!
Calling your boyfriend “brother” seems a little more than creepy in the Western world, but the term 오빠 implies admiration, and therefore, labelling your boyfriend as 오빠 shows that you respect him. In return, the boyfriend takes on the role as the protective older brother and is expected to take good care of his girlfriend.
Be sure to use these terms with caution though as you can only call someone that is older than you 오빠, 형, 언니 or 누나. By older, we mean someone who was born in a year prior to your birth. So, even if the person is a few months older but born in the same year as you, addressing him or her this way is a big no-no. That being said, although there isn’t a set age limit, addressing someone who is over the age of 40 with any of these labels seems a little strange and awkward.
So how do you refer to your younger siblings and close friends? Well, when talking about them, you can call them the following: