Augusta d'Aguillar Stewart

Augusta d'Aguilar Stewart (1783-1836), who married her distant cousin, John Stewart, son of William Stewart of Castle Stewart and the Hon. Euphemia MacKenzie, daughter of Kenneth Mackenzie, Lord Fortrose in 1804

   Frederica Stewart (1806-1887), who married Major Edward Shalders in 1833 and had issue.

https://en.wikipedia.org/wiki/Keith_Stewart

Please note: A genealogist in Scotland has checked Keith Stewart of Glasserton's will, and there is no mention of a daughter, only of his four sons.

Her birth conflicts with his eldest son, Archibald Keith Stewart, who was born in 1783 and died in 1795, when he fell off the Navy ship he was serving on and drowned.

I have found no birth or baptiim record.

I would have thought it was highly unlikely she was Keith Stewart's there as no mention of her in her alleged father's will, she should have been, as she was a minor.
There is also no mention of her in her mother, Georgina Isabella d'Aguilar's will, only the four sons are mentioned. It also mentions two of the sons wee deceased at the time of her death.

Also not only was Keith Stewart a distant cousin of William Stewart of Castle Stewart, Euphemia's mother, Mary Stewart, (Lord Fortrose, Kenneth MacKenzie's) as his elder half sister by nineteen years.

I have added information from Westminister Abbey, the write up (not the monument itself) states John Stewart died unmarried.

I have also added an excerpt from the book "reminiscences of many years, by Charles John Shore, 2nd Lord Teignmouth, who was married to Caroline Browne, daughter of Catherine Browne nee Stewart, Captain John Stewart's sister. This states also, he died unmarried.

There is no mention of any wife or daughter in his mother's will. His mother died in 1817, or of any of his sisters.

Jennifer Browne