American journalist and author H. L. Mencken (1880-1956) wrote in A Book of Burlesques (1916):
 
“Wealth. Any income that is at least $100 more a year than the income of one’s wife’s sister’s husband.”
 
The quotation has been frequently cited, often with “brother-in-law” replacing “wife’s sister’s husband.” The name “The Brother-in-Law Test” was used in a 1994 book. Mencken’s quotation illustrates that wealth is relative to one’s circumstances and environment.
 
 
Wikipedia: H. L. Mencken
Henry Louis “H. L.” Mencken (September 12, 1880 – January 29, 1956) was an American journalist, essayist, magazine editor, satirist, critic of American life and culture, and scholar of American English. Known as the “Sage of Baltimore”, he is regarded as one of the most influential American writers and prose stylists of the first half of the twentieth century. Many of his books remain in print.
   
Google Books
A Book of Burlesques
By Henry Louis Mencken
New York, NY: John Lane Company
1916
Pg. 209:
Wealth. Any income that is at least $100 (Pg. 210—ed.) more a year than the income of one’s wife’s sister’s husband.
 
Google Books
The Complete Book of Greed:
The Strange and Amazing History of Human Excess
By M. Hirsh Goldberg
New York, NY: W. Morrow
1994
Pg. 104:
THE BROTHER-IN-LAW TEST
How does one determine who has wealth? H. L. Mencken, the acerbic observer of human behavior and social commentator on America during the first half of the twentieth century, once offered his sure test of wealth. It is, he wrote, “any income that is at least $100 more a year than the income of one’s sister’s husband.”
   
Freakonomics
How Big of a Deal Is Income Inequality? A Guest Post (By William Bernstein.—ed.)
STEPHEN J. DUBNER
08/27/2008 | 1:07 pm
(...)
The fallacy of this argument is that human beings do not measure their well-being by absolute real income or longevity — but rather in relative terms. To paraphrase H.L. Mencken, a wealthy man is one who earns more than his wife’s brother-in-law.
 
The Wall Street Journal
Baby, You’re a Rich Man
You don’t have to be Warren Buffett to be considered wealthy. It all depends on who is setting the bar. Here’s why it matters for your taxes, investment options and college aid—and what you can do about it.
By LAURA SAUNDERS
Dec. 28, 2012 5:46 p.m. ET
(...)
Then there are emotional measures of wealth. H.L. Mencken once quipped that wealth in America means making a little more money than your brother-in-law.
 
Washington (DC) Monthly
March 04, 2013 3:00 PM
How Bad Is Wealth Inequality in America?
By Daniel Luzer
(...)
COMMENTS
theod on March 06, 2013 1:32 PM:
There’s a simpler reason for the unfathomability POV. People tend to hang out mostly with their peer group of class and income. If you don’t know rich people, you don’t have sense of their numbers (of any sort). You do, however, a a very finely tuned sense of your peers. That’s why Mencken said this about the definition of a rich man: “Your brother-in-law who makes a dollar a year more than you do.”
   
The Huffington Post
Wray Herbert (Author, “On Second Thought: Outsmarting Your Mind’s Hard-Wired Habits”)
Wealth, Welfare and the Brother-in-Law Rule
Posted: 09/10/2014 1:34 pm EDT Updated: 11/10/2014 5:59 am EST
(...)
The 20th-century journalist and social satirist H.L. Mencken once quipped that a wealthy man is one who earns $100 a year more than his wife’s sister’s husband.
   
BloombergView
Admit It: You’re Rich
JAN 7, 2015 1:52 PM EST
By Megan McArdle
(...)
But often, our comparisons are even more provincial. We judge our incomes against those of our neighbors, our colleagues, our family members—hence the old joke that a rich man is one who makes $1,000 more a year than his brother-in-law.