Hi Joanna, I have a younger sister but I don't have a younger brother. = 여동생은 있는데 남동생은 없어요. --> "-는데" means "but" in this sentence. Hope it helps.

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.