One of the very best K-Dramas I ever watched. I didn't mind the "bro and sis" thing because it was very clear - to me anyway - that he was in love with her from the very start. So she was never really his "sister" in his mind/heart. I also think she loved him in that way for the whole time, just didn't acknowledge it consciously.
If you like space K-movies/series, watch 'Space Sweepers' with Song Joong Ki and Kim Mu Yeol! That was the very first Korean show I ever watched, and it got me hooked. It's really good, funny, and emotional - with a surprise in the plot. 'Silent Sea' and 'The Moon' are also pretty good.