I think it would probably have to be “No, you shut up!!”
With a close second of “Stop laughing, that’s really our name.”
While “It’s taking longer than we thought.” probably applies, that is taken so my fallback would be “Buy low and sell high.”
“Non carborundum illegitimi est”
[Don’t let the bastards wear you down.]
“Go to sleep!”
Literally I think that comes out as “bastards aren’t grinding”. 
Well, I got it from my Dad, and he picked it up in WW2. So, yes, it’s mock Latin.
and, cite: Illegitimi non carborundum - Wikipedia
We descend from the same clan.
On the mail order family crest plaque that’s ours, but really not because my family’s Polish name was anglicized when they emigrated: Nil Desperandum.
Dad’s side: Miseris succerere disco
My family’s actual, oft repeated words to live by: If you wore shoes it wouldn’t happen.
“We won’t leave bruises or visible marks.”
“If you can’t say something nice about someone…sit next to me”
[sub]yes, we stole it. we’re not an original bunch[/sub]
“Don’t ever spend money on fancy luxurious stuff.”
Yes, I was raised by Depression babies.
I often think of this as my mother’s personal motto. Makes her a blast at parties. 
My mother says it must be, “Never miss a meal.” She hates the habit on my dad’s side of the family of discussing what we will eat at the next meal while we are eating the current one.
We have a few of those. Some people eat to live, and some live to eat.
pontis transimus quom pervenimus
On my dad’s side, it’s Veritas Vincit (Truth Conquers) and on my mom’s it’s Non Oblitus (Not forgetful).
For more practical applications, I’ll probably just steal from a good friend and say, “Sometimes shit just happens.”
Because we’re a fucking family, that’s why.
The two best parts of the day are dinner and a book.
I love my family.
Never Forgive,
Never Forget,
Imperfection Makes You Worthless.
Now I do, too. That’s a motto I can solidly get behind.