I read this at least a dozen years ago. What I remember: there is a young girl (possibly adopted, but definitely less parentally favored) who tries to save her sick younger sister with doses of dragon's milk. To get the milk, she makes a bargain with a dragon. But after the mother dragon is killed, Kaeldra takes on the responsibility of the three baby dragons left behind. She and a would-be dragon slayer traverse a surprisingly well-thought-out fantasy world, trying to keep ahead of their enemies.