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With allusion to Cinderella’s ugly and unpleasant stepsisters in the fairy tale Cinderella, the phrase ugly sister(s) (also with capital initials) designates a person (or persons) or a thing (or things) considered unattractive, inferior or unpleasant, compared to others of the same type or group.

This phrase occurred, for example, in a transcript of the speech that the British politician Roy Jenkins (1920-2003), of the Social Democratic Party (SDP), delivered on Monday 15^th September 1986—as published in The Daily Telegraph (London, England) of Tuesday 16^th September 1986 [page 8, column 7]:

The SDP did not wish to grow up like one of the ugly sisters of British politics, Mr Jenkins said.
“If we merely turn the two-party system into one with three peas in the pod, we shall almost inevitably be number three.
“We must be an anti-party party, seeking a different relationship with the electorate—particularly the non-partisan electorate—from that of the dreadful two.”

These are, in chronological order, the earliest occurrences of the phrase ugly sister(s) that I have found:

1-: From the following letter to the Editor, by ‘Pimlicola’, published in The Times (London, England) of Wednesday 21^st May 1851 [page 5, column 2]:

Sir,—When I last addressed you everything seemed dismal and disgusting. The Whigs wouldn’t govern us, the Protectionists couldn’t, the Peelites neither could nor would. As for the Manchester mind, it sat like Cinderella in the dust-hole, and biding its time, and thinking of the “coming man” while its ugly old sisters the rival factions were fighting for the slipper.

2-: From A Savage Beauty ¹, published in the Lancaster Daily Intelligencer (Lancaster, Pennsylvania, USA) of Thursday 3^rd June 1869 [page 1, column 4]:

The night fell suddenly down, dark and windy; the tide was at its highest, and only the extreme tips of the “nipas”—that ugly sister in the graceful family of palms—rose above the flood.

¹ This text, A Savage Beauty, by the British author Frederick Boyle (1841-1914), was republished in Camp Notes. Stories of Sport and Adventure is Asia, Africa, and America (London: Chapman and Hall, 1874).

3-: From Kind Suggestions to the Charity Commissioners, published in The Brooklyn Daily Union (Brooklyn, New York, USA) of Saturday 23^rd August 1873 [page 2, column 2]:

The sum and substance of the Eagle’s allegations relative to the Charity Commissioners is in plain terms that they are an unprincipled set of Democratic scoundrels. […]
Now, we started out with a mission of charity in view for these Charity Commissioners. The Eagle, acting through Mr. Cunningham, has agitated the public as well as the paupers on the policy of impolicy of peas. When a requisition for rice comes before the Commissioners, Cunningham cunningly calls for peas. He affirms, with the confidence of a Blot, that they are better for the eleemosynary stomach than rice; they can be had for a shilling a bushel […].
[…]
[…] The great thing is to find the food sufficiently rich in nitrates, as the carbonates are easily to be had. Now of thirty or forty kinds of food known to contain this needed element pease comes third in the list, while rice is actually the tenth. To get your five ounces of nitrates, you require only one pound nine ounces of pease, while of rice you will need exactly five pounds.
All the Charity Commissioners have to do, then, to have everything turn out, as nicely as the end of a pretty story, is to order those seven hundred bushels of pease. They will have builded better than they knew. Cunningham will have unconsciously conferred a culinary blessing on pauperdom; the Eagle, like the ugly sisters in the story, will have come to grief, and Tʜᴇ Uɴɪᴏɴ will be the little bird that helped the Commissioners to save their damaged reputations and ‘pick up the pease.’

4-: From Georgetown Society. The Changes of a Hundred Years, by ‘Olivia’, published in The Times (Philadelphia, Pennsylvania, USA) of Friday 22^nd October 1875 [page 1, column 4]:

During the Presidential reign of these two men ² the social queen of the capital ³ lived in Georgetown, the city of her birth and education. […] She had standing at her right hand the incomparable Harriet Lane ⁴, of the White House, who held the last royal sceptre of this extinct line. History rarely records the fact that distinguished leaders are beautiful, but popular acclamation gave to both these women the fairest crown. […] In the days which marked the magnificence of the Bodisco and Lane regime, beauty and grace were not punished as under the Grant ⁵ dynasty. George H. Williams ⁶, of Oregon, would have been Chief Justice of the Republic to-day had his wife been one of the ”ugly sisters.” “They pared their heels and they pared their toes,” but the Prince did not dare defy the “Sisters.” Underneath the political drift lies the stony social strata which decides the character of the products above.

² This refers to James Knox Polk (1795-1849), 11^th President of the USA from 1845 to 1849, and to James Buchanan (1791-1868), Secretary of State from 1845 to 1849, and 15^th President of the USA from 1857 to 1861.
³ This refers to Harriet Beall Williams (1824-1890), of Georgetown, District of Columbia, who, in 1840, married Alexander de Bodisco (1786-1854), the Russian ambassador to the USA from 1837 to his death.
⁴ This refers to Harriet Lane (1830-1903), who acted as First Lady of the USA during the administration of her uncle, James Buchanan—cf., above, note ².
⁵ This refers to Ulysses S. Grant (1822-1885), 18^th President of the USA from 1869 to 1877.
⁶ This refers to George Henry Williams (1823-1910), who resigned in April 1875 as Attorney General of the USA after his wife was accused of taking bribes.