Families have in-jokes. Something Mom or Dad always says in a particular situation; a remark no one else would catch but the parents and kids look at each other knowingly. They're not always funny. In fact, sometimes they're groan-inducing. But everyone would miss them if they stopped.
Many of my favorites are borrowed from pop culture and films. For example, I can't leave our car in a multi-story parking garage without saying, "Everybody remember where we parked." The twist on one of the best lines from one of the best Star Trek movies is that it was itself a riff on the cliched command of families everywhere. That's why it was funny: it's one thing when Dad says it at the mall, quite another when Captain Kirk says it to his crew as they debark from an invisible spaceship. The fact that I've re-repurposed it to again apply to Dad in a parking lot is deeply meta, man.
Bonus points to this movie for "double-dumbass on you," which I don't use as often as I'd like. Also for "nuclear wessels" because one of my daughters works where they used to keep them.
"Important safety tip. Thanks, Egon." From Ghostbusters, this applies in real life both when advice is unnecessary ("look before you cross the street") and when it's woefully inadequate ("in case of major earthquake, avoid falling debris").
"Have fun storming the castle!" from The Princess Bride. Applies to the start of any endeavour, but best for one involving some distance or difficulty. This got used a lot when sending the daughters back to college after a break. Most effective when my wife and I hug and wave like Billy Crystal and Carol Kane.
"Not anymore." From The Pink Panther Strikes Again, best pronounced with an outrageous French accent. Applies to any situation in which something has been botched or destroyed, and someone utters a sentence that begins, "But that's . . ."
And I can't think of any word that's been more damaged by a single movie than "surely":
What're yours? Or is it just us? It's just us, isn't it . . .