There is no evidence of a first marriage. Do you know any details?
And it does appear that his children were illegitimate.
The person who provided this material - many years ago - obviously didn't know about Olivia Grace.
I don't know her birth date but I have found out some other info.
Olivia Stuart married George Thomas Gowan at Calcutta, 1854. (Source: Women in India)
She was probably about 21. Her brother was born in 1835, which would make him 19 at this time.
In ‘Narrative of Events Attending the Outbreak of Disturbances and the Restoration of Authority in the District of Moradabad in 1857-1858’ [the Indian Mutiny] by J.C. Wilson Esquire, aka Cracroft Wilson, Judge and Sessions Judge of Moradabad. He makes considerable reference to a Captain Gowan of the 18th Regiment Native Infantry.
Henry Benson Stuart (Olivia’s brother) was in the same regiment in 1854/55. (retired as a captain)
Robertson & Co.'s Bengal almanac, companion, and ... 1853.
EDITED BY C. U. WILSON, M.A., OF THE BENGAL EDUCATION SERVICE,
List of Inscriptions on Tombs or Monuments in Bengal possessing Historical or Archaeological Interest. Calcutta, 1896
St Paul’s Cathedral. Consecrated 1847.
June 1857. George Thomas Gowan.
To the beloved memory of GEORGE THOMAS GOWAN, Captain in the 27th Regiment Bengal Native Infantry, and Commandant 9th Regiment, Oude Irregular Force, second son of Major-General G. E. Gowan, Bengal Artillery, killed at Sitapur, Oude, June 1857, in the 35th year of his age. He fell whilst endeavouring to recall the mutinous sepoys of his own corps to order and obedience.
June 1857. Olivia Grace Gowan and infant
Also to the memory of OLIVIA GRACE, daughter of Major-General James Stuart, Bengal Army, and wife of Captain G. T. Gowan, who, with their infant son, George Boyce Combe, was cruelly murdered by the rebels at Sitapur, June 1857.
Fascinating, but very sad.