This sounds like  [Fifth Quarter]( https://www.goodreads.com/book/show/175312.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.