My husband only met my grandfather once, before he died, and while he accompanied me to my grandfather's funeral, I would argue that he could not even truly miss my grandfather as he never really got a chance to get to know him, even though he DID meet him.
My husband's parents have both passed (same day, two years apart). I only met his father a couple of times, whereas I knew his mother much better. I don't really feel like I can say that I miss my father-in-law, but I do miss my mother-in-law.
And actually, if/when Cathy passes, it would be totally OK for Jill to post about how much she misses her. At this point, they have formed their own relationship independent of Derick.