I have a younger sister but I don't have a younger brother
저는 여동생은 있지만 남동생은 없어요.
Basically the word 'but' means 그러나(or 하지만) in korean.
있지만 = 있다 + 하지만 = BUT
In addition..
있는데 = 있다 + 그런데 = AND or BY THE WAY
so, if you want to say the word 'but' in korean you'd better say 하지만 or 그러나.