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.
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 10 years older than you with any of these labels seems a little strange and awkward.
It all depends on the person you’re addressing though, as some much older people like being address this way. For example, I’m 26 and my older female hairdresser who is in her early 50s just loves it when I call her 언니. If you’re in any doubt, there is no shame in asking what they’d prefer you to call them.
If 오빠, 형, 언니 or 누나 isn’t appropriate to use, such as when talking to someone who is more than a couple of years older than you, then you might find 씨 more natural. This is something called a polite suffix, and it is attached directly to someone’s name to show respect to the person you’re talking to. It is often compared to how the titles ‘Mr’, ‘Misses’, or ‘Miss’ are used in English; although 씨 is placed after the name, rather than preceding it, and is used a lot more frequently.
We can use this suffix by attaching it to a person’s given name, or full name to show the most respect. You should avoid applying it to just a surname, as this can be seen as rude and disrespectful.