I don't think you should be able to cast Speak with Dead on Kanon more than once (if that's what you mean by recast), but I do think it would be nice if you could give his sister some closure with the spell if you get there early enough, while she's still grieving over his body but before she runs off to take care of Sazza.
That said, this is somewhat related...
I wish Speak with Dead worked on Kanon's sister when you later find her body along the trail to the goblin camp.
