Another family connection too; have you come across the name Lockhart yet? Ann Sinclair's mother was Martha Lockhart. In some of the correspondence between the Bogles and William Sinclair, a Robert and a John Lockhart are refered to as "brothers". I think they were also merchants, possibly London or Kingston (or both).
There was a Sinclair branch of the Stevenson line that changed their name to Lockhart in the mid 1700's. James Sinclair assumed the surname of Lockhard on succeeding his uncle, Lord Woodhall of Castlehill. There was Martha (see above) of that line, either grand-daughter or great-grand-daughter of James Sinclair/Lockhart.