I'm pretty sure I read this one in the early to mid 90s in Kentucky. It was a paperback fantasy novel. I think it had the girl leaping off of the ramparts of a castle and starting to develop wings (or maybe silhouetted by a bird shape). The basic plot line was the children are in a castle besieged by a man (I honestly don't remember if he was evil, ambitious, or why he was trying to take their castle). I don't remember where their parents were, or if they were still alive. The girl can shape-shift, but largely has to conserve mass, I think.
Near the end of the book, the besieger agrees to let the girl go, but refuses to let the boy go, so she looks up a spell that lets her turn her brother into a sword so that she can sneak him out, the spell being reversed by her immersing the blade in the sea. For some reason, she winds up using the sword in a fight, impaling one of the besieging army (maybe the leader?) in the heart. This results in her brother transforming back (human blood being similar enough to sea water), but she is still allowed to leave with him because technically speaking, she only left with a sword.
I want to say that the title was Taking Flight, but I think I might be confusing that with the Lawrence Watt-Evans Ethshar novel, which I read around the same time.