I only just came across your message.I am descended from the Fairweathers of Angus too.
Your Agnes Fairweather is the daughter of George Fairweather and Agnes Langlands.George is the son of David Fairweather (b. 1827, Kirriemuir) and Isabel Hood (b. 1735, Angus).David in turn is the son of James Fairweather.
I am descended from James' other son, Alexander (b. 1825, Angus).