It’s “the Melcherses.” Really.
I talked a bit about pluralizing proper nouns ending in “s” in this post the other day. It really trips people up.
Here’s a local surname that causes a lot of trouble.
Family name: Melchers. This is the singular proper noun. Note that the name is not “Melcher.”
Still singular (two individuals, same last name): Gari and Corinne Melchers lived at Belmont.
Plural: The Melcherses lived at Belmont.
Singular possessive: Gari Melchers’ studio was a cozy place.
Plural possessive: Belmont was the Melcherses’ home.
We mostly get it right in the paper, though in one notorious example a few years ago we got it spectacularly wrong. This happened despite the painstaking efforts of the copy editor, who made sure the singular, plural, singular possessive and plural possessive were formed correctly throughout the story.
Another editor saw it on the page proof, decided the plural “the Melcherses” looked funny, and marked it to be changed it back to “the Melchers.” (To WRONG, that is. Remember, the singular name is “Melchers,” not “Melcher.”) To further muck things up, the correct plural possessive, “the Melcherses’ Christmas,” got changed back to “the Melchers’ Christmas.” A cornucopia of wrongness!
Ugh. I hope this won’t happen again. Can’t promise, of course.