This sounds like Fifth Quarter by Tanya Huff.
It was published in 1995 and the blurb matches your description:
brother and sister assassins
brother body is stolen and his spirit is forced to share sister's body
hunting for brother's body
It has been a long time since I read it, so I don't know if it has the bit regarding the broken wrist. The cover art is a similar style to Mercedes Lackey's works, so that is another match.
This is the second book of the series but the first with these characters.