"That Term is Not Currently Favored..."

My sister recently converted to Judaism—an event that was broached tactfully by my parents and sister to my grandfather, an elderly and old-fashioned gentleman. Tonight at dinner, my grandfather says, "So is Jane a Jewess now?" The rest of the dinner party remained silent, grasping for the correct answer until I tell him, "that term is not currently favored....I think people prefer 'Jew.'"
"But what do you call a male Jewish person?"
"A Jew."
"So you would call a female Jew a Jewess."
"No," I say, "No more than you would call a female Christian a Christianette."

In the end, Grandpa was not satisfied with our answer that, for some unknown reason, the female form of the word "Jew" was offensive.