The Honourable Augusta Maria Leigh (née Byron; 26 January 1783 – 12 October 1851) was the only daughter of John "Mad Jack" Byron, the poet Lord Byron's father, by his first wife Amelia Osborne (Lady Conyers in her own right and the divorced wife of Francis, Marquis of Carmarthen).
Augusta's mother died soon after her birth. Her grandmother, Lady Holderness, raised Augusta for a few years, but died when Augusta was still a young girl, and the child divided her time among relatives and friends.
Augusta later married her cousin, Lt. Colonel George Leigh (1771–1850), son of General Charles Leigh (1748–1815) and his wife Frances Byron, her paternal aunt. She had seven children by him. Richard Temple-Nugent-Brydges-Chandos-Grenville, 1st Duke of Buckingham and Chandos, noted the wedding with disdain in his diary: "Poor Augusta Leigh marries today! Poor thing! I pity her connecting herself with such a Family, and such a fool! However she has nobody to blame but herself." The marriage turned out unhappily, because he fell into dissolution and gambled away all his money. At the end, he left his wife and children nothing but debts. 
There was personal unhappiness too, when Henry Trevanion, husband of one daughter Georgiana left her in 1829–30 (after four years of marriage) for her younger sister Elizabeth Medora (allegedly fathered by Lord Byron). Medora and Henry Trevanion remained together for several years in France, before finally separating. The deserted Georgiana, doubly betrayed, lacked the funds to apply for a Parliamentary divorce unlike another similarly wronged wife Louisa Turton (Turton vs Turton).
Augusta's half-brother, George Lord Byron, didn't meet her until he went to Harrow School and even then only very rarely. From 1804 onwards, however, she wrote to him regularly and became his confidante especially in his quarrels with his mother. Their correspondence ceased for two years after Byron had gone abroad, and was not resumed until she sent him a letter expressing her sympathy on the death of his mother, Catherine.
Not having been brought up together they were almost like strangers to each other. But they got on well together and appear to have fallen in love with each other. When Byron's marriage collapsed and he sailed away from England never to return, rumours of incest, a very serious and scandalous offence, were rife. Some say it was because of his fear of prosecution that Byron abandoned his country.
There is some evidence to support the incest accusation. Augusta Leigh's third daughter, born in spring of 1814, was christened Elizabeth Medora Leigh. A few days after the birth, Byron went to his sister's house Swynford Paddocks in Cambridgeshire to see the child, and wrote, in a letter to Lady Melbourne, his confidante: "Oh, but it is not an ape, and it is worth while" (a child of an incestuous relationship was thought likely to be deformed).
"Medora" is the name of one of the heroines in Byron's poem The Corsair, which was written at Newstead Abbey during the three weeks in January 1814 when the poet and a pregnant Augusta were snowbound there together. However, Augusta's husband, George, never questioned the paternity of Medora, and she grew up among her brothers and sisters unaware that she might be the first of Byron's three daughters.
In fact, they were entertained by his in-laws at the family home in Leicestershire for several weeks after Byron had married Annabella Milbanke. At that time Augusta wrote to her sister-in-law about Medora, saying: "The likeness to Byron... makes her very good-humoured". In another she wrote, knowing it would be shown to Byron, "Here comes Medora".
Medora did become aware of her possible paternity years later and she and her child were assisted financially by Augusta Ada Byron, who was also a source of emotional support when Medora fell on hard times. Neither Medora nor her mother met Byron's daughter by Claire Clairmont, Allegra Byron, who died at age five in 1822 in an Italian convent.
Medora had 6 (possible half) siblings by her mother and her husband Colonel George Leigh of the 10th Dragoons. One of her sisters, Georgiana Augusta Leigh, married Henry Bettesworth Trevanion. The marriage between Georgiana and Henry was not harmonious and Medora was often used as a "chaperone". Medora's life became more complicated as a result.
Medora's father, Colonel Leigh, subsequently discovered that she was pregnant with a child by Henry Trevanion (her brother-in-law). Colonel Leigh sent Medora to an establishment in Maida Vale, London, where upper-class girls went to have their illegitimate offspring. Henry arranged for her escape.
Henry Bettesworth Trevanion and Medora went to Normandy, where her child was stillborn. However, Henry was so infatuated with Medora that he wanted a living child by her.
Henry ran away again with Medora and prevailed on her to set up in an ancient, tumble-down chateau near Morlaix in France. There they lived as brother and sister and passed as such, for they looked very much alike. With no idea of money on either side they were reduced to poverty; as aristocratic delinquents they never considered turning their hands to work. In exile they used the surname AUBIN.
By 1833 Henry and Medora were living in Brittany, at the Breton Carhaix.
Medora became a Catholic and declared her intention of entering a convent. However, she got pregnant again by Henry. The Abbess was tolerant and found Medora lodgings outside the convent, where a living child was born on 19 May 1834; she was baptised Marie Violette Trevanion on 21 May 1834. [CHRISTENING: See page 116 of "Byron's Daughter" by Catherine Turney (1975) biography of Elizabeth Medora Leigh]
Due to poverty and illness, the pair eventually had to beg their families for money. Henry's father, Major John Purnell Bettesworth Trevanion of Caerhays Castle, Cornwall, thought Medora was to blame for the situation. He sent one of Henry's uncles to Brittany to persuade Henry to return to England. Henry refused to leave. Augusta Maria (Byron) Leigh was now keeping her other daughter Georgiana's three children by Henry, but sent what money she could to Medora. However, Augusta eventually lost touch with Medora, who had become ill in Brittany after a series of miscarriages.
In 1838, Henry Trevanion and Medora Leigh finally parted permanently. In an autobiography, Medora later wrote of Henry that he "gave himself up to religion and shooting". Henry died in 1855 in Brittany, France.
Medora left for the south of France with her daughter Marie Violette, who later entered a convent and became known as Sister Saint Hillaire. Marie Violette is said to have died within the order she joined in 1873.
Medora travelled in poverty and eventually met Monsieur Jean-Louis Taillefer whom she married in Aveyron, France on 23 August 1846. In 1846 they had a son, Elie Taillefer, who lived until 22 June 1900. Marie Violette took the surname of her stepfather and became Marie Violette Taillefer.
Medora died on 28 August 1849 at Lapeyre, Aveyron, southern France, and is believed to be buried there.
Augusta is also the subject of Byron's Epistle to Augusta (1816) and Stanzas to Augusta.