Poderia este ser o livro de Elisabeth Waters, Changing Fate (1994)?
O resumo da Goodreads:
Akila uses her wits and her shape-changing abilities to save her brother Briam when their home is besieged. But that leaves them both on the run, with Akila trying to keep them safe, while Briam finds more trouble to get into. When he is chosen as year-king in the city of Diadem, Akila struggles to save his life, finding help in the most unexpected places.
Um Resumo da Biblioteca :
Acila has run Eagle's Rest for her twin brother Briam since their father left on another land grabbing expedition. When Lord Ranulf comes bearing bloody proof of their father's death to take their land and home Acila is determined to save her brother and get them both far away from the usurper. However, Lord Ranulf isn't just interested in a kingdom anymore because he now knows Acila's secret which gives him a particular incentive to marry her.
Ao procurar nos livros do Google ( minha pesquisa ), encontrei esta citação (página 169):
They say that the lady there was a powerful sorceress, and she turned herself into a magic sword so that her twin brother could use her to kill a man in single combat and win his freedom. Then she changed back, bathed in her victim's blood — and my father asked her to marry him!
A capa da edição original da Goodreads pode ser o que você lembra:
Obviamente, isso tem algumas diferenças em relação ao que você disse, mas muito perto.