Was your sim friends with the two sims before they died? And still friends with them now? I had this happen to me once, where one of my sims' father had died accidently, and the relationship bar had decayed too low and I hadn't noticed. I waited until the ghost appeared, had my sim socialise with her father, rebuilt their friendship. Then the wish worked.
If the wish is still grayed out, it might be a mod issue. I don't know anything about that, but I'm sure someone else could offer some solutions.