I can see why they ended it the way they did. It would've been a pseudo happy-ending otherwise what with Yi Soo turning himself in and being imprisoned for murder.
For a moment, I thought Yi Soo's sister's terminal illness was very contrived, a desperate move on the writers' part to give the show a dramatic twist at the eleventh hour. But I think her illness was foreshadowed as early as episode10 or when she had those nose bleeds.
So, all in all a very good drama.