We're descendants of Arch. McGrigor and Nancy/Ann McLean:
In the 1851 Canada Census, South Dorchester, Elgin County, Ontario, Peter McGregor (b. 1781, Scotland) and his wife Mary MacArthur-McGregor (b. 1792, Scotland) are living with their family in Canada (some were married with children) some were single. Nothing on Margaret McGregor, b. 1825, in Keils, North Knapdale, Argyll, Scotland for that Census (she must have been living elsewhere) however, if Gregor and Archibald were her sons, they were living with Peter and Mary in South Dorchester:
Grandson , Gregor McGregor, Born Canada, Presbyterian, age 7, year 1845; Grandson, Archibald McGregor, Born Canada, Presbyterian, age 4, year 1848.
In closing, Margaret McGregor, was daughter of Peter and Mary McGrigor/McGregor - Archibald (married to Ann/Nancy McLean) was her brother.
Feel free to write if you wish to discuss this further.