The phrase if your, or if my, aunt had been your, or my, uncle, she’d have been a man (also if your, or if my, aunt had been a man, she’d have been your, or my, uncle), and variants, are used to rebuke someone who has used an unrealistic conditional.

These are the earliest occurrences that I have found, in chronological order:

1-: From a letter, by a person signing themself ‘Harry Homespun’, published in The Republican; and Savannah Evening Ledger (Savannah, Georgia) of Saturday 20^th August 1808—‘Harry Homespun’ was reacting to a letter, by ‘R.’, which had been published in the Columbian Museum and Savannah Advertiser (Savannah, Georgia):

Some gentleman, who signs himself R. (by which, I suppose, he means Right) […] appears to be a little infected with the mania, so prevalent in certain circles, viz. the fear of a French invasion […].
[…]
Why, Mr. R. […], do not be uneasy, the Corsican ¹ will not eat us yet. […] Bonaparte is letting blood 300 miles distant from us. We pity the fate of his victims; but yet can see no necessity to be alarmed at his approach, so long as he makes retrograde movements. Even you have acknowledged that he can’t get here so long as British oak floats upon old ocean.
Since, then, this barrier is sufficiently strong in itself, we, at any rate, have nothing yet to fear. But if this barrier should be broken down, what then would become of us? Aye, true. But if my aunt had been my uncle, what would have been her gender?
That IF is a most formidable word, and often stands as a stumbling-block in the way of many ambitious men.

¹ Napoléon Bonaparte, Emperor of the French as Napoléon I from 1804 to 1814 and in 1815, was born in 1769 and died in 1821.

2-: From R.’s response to Harry Homespun’s letter, published in the Columbian Museum and Savannah Advertiser of Tuesday 30^th August 1808:

For Wit—O! in this you are above all things splendid!—“If your aunt had been your uncle, what would have been her gender?”—This riddle puzzled me so much that I read it to M—k—y J—k, as both he and you are very clever in your own way—and what do you think was his answer—“Ah! massa” says he “she would surely have been a Hermaphrodite.” The answer I think was about as bright as the question.

3-: From the Norwich Aurora (Norwich, Connecticut) of Wednesday 18^th December 1839—these two paragraphs were reprinted in several U.S. newspapers in December 1839 and in 1840:

If Gen. Harrison ² can carry Pennsylvania, New-Jersey, New-York, Massachusetts, Vermont, Connecticut, Rhode-Island, Ohio, Indiana, Michigan, and Kentucky, he will be elected.—Madisonian.
If your aunt had been a man, she would have been your uncle, squire Allen ³.—Bost. Post.

² William Henry Harrison (1773-1841) was a U.S. military officer and politician who served as the 9^th President of the United States in 1841.
³ I have not found out whom “squire Allen” refers to.

4-: From Chapter VI. Tom and Sarah, in The London Edition of The Mysteries of Paris, published in the Boston Daily Times (Boston, Massachusetts) of Monday 30^th October 1843:
—Tom and Sarah, who is disguised in man’s apparel, have arrived at a flash-ken (i.e., a house frequented by thieves); they sit themselves down at the Slasher’s table while the hostess is serving wine:

The Slasher […], speaking to the Ogress, ‘Capital stuff this wine.’
‘That’s one reason your glass should not stand empty,’ said Tom, at the same time refilling it.
‘Here’s your health, my cove, and that of your little friend who—that is—why—if my aunt had been a man, she would have been my uncle, as the old proverb goes.’
Sarah blushed almost imperceptibly.

This is the corresponding passage from Chapitre VI. Thomas Seyton et la Comtesse Sarah, in Les Mystères de Paris (Paris: Charles Gosselin, 1843), by the French author Eugène Sue (Marie-Joseph Sue – 1804-1857):

Le Chourineur […]—Voilà de fameux vin, mère Ponisse !
—C’est pour ça qu’il ne faut pas laisser votre verre vide, mon brave—reprit Thomas Seyton en versant de nouveau à boire au Chourineur.
—A votre santé—dit celui-ci—et à celle de votre petit ami qui… enfin suffit… Si ma tante était un homme, ça serait mon oncle, comme dit le proverbe… Allez donc, farceur !… je m’entends.
Sarah rougit imperceptiblement.

5-: From The Pittsfield Sun (Pittsfield, Massachusetts) of Thursday 10^th December 1846:

Correspondence of the Boston Post.
New-York, Dec. 5, 1846.
The N. Y. Journal of Commerce of yesterday contained a letter from some one in Boston, in behalf of the Hon. Abbott Lawrence, in which something in the shape of an apology was made for the extravagant predictions that were made by that eminent gentleman, anticipatory of ruin and specie suspensions, in his letter to Mr. Rives of Virginia. The writer, who is apparently very ready to concede that Mr. Lawrence was exceedingly unwise when he made his ruin-stop specie-payment predictions, says that if he had supposed, at the time he committed the deed, that a famine would visit Europe, he most assuredly would not have hazarded so wild a vagary. “If your aunt had been a man, she would have been your uncle.” Does anybody doubt that? No, no man living doubts it, and no man believes that Mr. Lawrence, at the time he made his hardy prognostication, thought that it would be verified. Nor did he, I fancy, dream that a year or eighteen months would pass away so very rapidly and bring no ruin with them.

The earliest occurrence that I have found of the variant with uncle as the grammatical subject is from The Causes of Johnston’s Defeat, published in the New-York Daily Tribune (New York City, N.Y.) of Monday 20^th October 1851:

When a matter has gone wrong, there is not much use in undertaking to show how it might and should have been otherwise. “If your uncle had been your aunt” there’s no telling what changes would have been consequent upon this one—and not much use in telling.

The following are texts in which occur some of the other variants of the phrase:

1-: From the column Putting ’Em Over: Rabid Rudolph Says, published in the Muskogee Daily Phoenix (Muskogee, Oklahoma) of Sunday 6^th August 1916:

If Benny Leonard hadda knocked Fred Welsh for a field goal, Ben would have been champion, and if your aunt had whiskers she’d be your uncle.

2-: From Fallen Bastions: The Central European Tragedy (London: Victor Gollancz Ltd, 1939), by the British journalist and author George Eric Rowe Gedye (1890-1970)—the five circles, of course, replace the letters of the noun balls:

If only—“If only”, a certain outspoken and impatient member of the London diplomatic corps is supposed to have said recently to Lord Halifax ⁴ when the Foreign Secretary had piously murmured something about “If only Hitler would back his will to peace with deeds”—“If only your aunt had ● ● ● ● ●, she would be your uncle.”

⁴ The British Conservative politician Edward Frederick Lindley Wood (1881-1959), 1^st Earl of Halifax, served as Secretary of State for Foreign Affairs from February 1938 to December 1940; he pursued a policy of appeasement with Germany.

3-: From the review by Jack Balch of the U.S. war film Objective Burma, published in the St. Louis Post-Dispatch (St. Louis, Missouri) of Friday 9^th March 1945:

George Tobias ⁵, as one of the paratroopers, sums up the stubborn admirable quality in this portrait of men up against it and determined to come through. When one of his comrades, beginning perhaps to crack, day dreams dangerously about how things “ought” to be, George says, “Yeah, and if your aunt had a beard, she’d be your uncle.” This restores perspective.

⁵ George Tobias (1901-1980) was a U.S. actor.

4-: From Slumping, hurt Expos take on Padres tonight, by Brodie Snyder, published in The Gazette (Montreal, Quebec) of Friday 24^th August 1973:

Manager Gene Mauch says: “If we can win 21 of those 30, we’ll be right there. If I could pick the 21 we’d win. I know we’d finish first.”
Yes, Gene, and if your uncle had different equipment, he’d be your aunt.

5-: From Cyclone fans are ready for a second helping of magic, by Marc Hansen, published in The Des Moines Register (Des Moines, Iowa) of Sunday 11^th October 1998:

“In the second half we got complete control of the line of scrimmage,” Smith said. “We might have had two or three more scores if we hadn’t stopped ourselves.”
If, if, if. Here we go with the ifs again. And if your aunt had a Y chromosome, she’d be your uncle.

 

IN FRENCH

 

It seems that it is in the above-quoted passage from Les Mystères de Paris that is found the earliest occurrence of the French phrase si ma tante était un homme, ça serait mon oncle, meaning, literally, if my aunt were a man, that would be my uncle.

A variant, si ma tante en avait, je l’appellerais mon oncle (if my aunt had any [i.e., if my aunt had balls], I would call her my uncle) occurs in La Guerre des Soldats: Le Champ d’honneur. Conseils de guerre aux armées. L’Hôpital (Paris: Flammarion, 1919), by the French Communist authors Raymond Lefebvre (1891-1920) and Paul Vaillant-Couturier (1892-1937).

However, the phrase, in some form or another, is already implied in the following song title, from Extrait du catalogue général des œuvres de chant des membres de la Société des Auteurs, Compositeurs et Éditeurs de Musique (Paris: Agence générale de la Société des Auteurs, Compositeurs et Éditeurs de Musique, 1866):

Si ma tante en avait ! [words by Jacquart, music by Couplet, published by Vieillot]

Another song, titled Si ma Tante en avait ! Ou “Si ma Tante ?”…., words by Louis Bousquet (1871-1941), music by Georges Picquet (1865-1940), was published in Paris, in 1914, by L. Bousquet.

The French journalist, historian and lexicographer Charles Virmaître (1835-1903) recorded—but misunderstood—the phrase in Dictionnaire d’argot fin-de-siècle (Dictionary of end-of-century slang – Paris: A. Charles, 1894):

Si ma tante était un homme.
Cette expression est employée communément dans le peuple pour exprimer l’absence de la virilité de la femme :
—Si ma tante en avait elle serait colonel dans la garde nationale (Argot du peuple).
     translation:
If my aunt were a man.
This expression is commonly used among the people to express the absence of virility in women:
—If my aunt had any she would be a colonel in the national guard (People’s argot).

One G. Barrier had given a much better interpretation of the phrase in Le rattachement des vétérinaires militaires au service de santé (The attachment of the military veterinaries to the health service), published in Recueil de médecine vétérinaire (Compendium of veterinary medicine – Paris: Asselin and Houzeau) of Sunday 15^th October 1893:

Cette façon de raisonner fait, malgré soi, songer à l’irrévérencieuse réplique de Gavroche ⁶: « Ah ! mais, si ma tante…. en avait, elle pourrait être de la Garde nationale ! » Avec des « qui nous dit », des « mais », des « si », il est facile d’échafauder les objections les plus invraisemblables, de noircir ce qui est blanc, de blanchir ce qui est noir.
     translation:
This reasoning makes, in spite of oneself, think of Gavroche’s ⁶ irreverent reply: “Ah! but, if my aunt…. had any, she could be of the National Guard!” With “who-tells-us-s”, with “buts”, with “ifs”, it is easy to construct the most implausible objections, to blacken what is white, to whiten what is black.

⁶ Gavroche, the name of a boy who lives on the streets of Paris in Les Misérables (1862), a novel by the French poet, novelist and playwright Victor Hugo (1802-1885), has come to be used in the generic sense of a Parisian street urchin.