My g-g-g-grandfather was John McTavish married to Elizabeth (maiden name unknown) who is said to have served with the Scottish Army for 7 years possibly with the Dragoons in the Irish Rebellion of 1798 before emigrating in about 1803-04 first to Nova Scotia and then to North Esk Parish, Northumberland Co.,New Brunswick (before NB was a part of "Canada".)
The evidence I have strongly suggests that John was from Inverness-shire and his wife was Irish.Unfortunately, I do not know his parents' names nor the specific parish he was from nor when or where he and Elizabeth were married.
John and Elizabeth's oldest daughter was Jane who married Patrick Malloy/Meloy.The oldest son was my g-g-grandfather, Duncan, b. 04 Jan 1804.in Nova Scotia.According to the Family Bible of this son Duncan and his wife, Anna Frances Flaherty McTavish, John McTavish died 20 Mar 1852 at the age of 84.His wife Elizabeth died 24 Dec. 1864 at the age of 87.
John and Elizabeth had additional children Catherine b. abt 1807; John b. abt 1809; Donald; Tavis; William b. abt 1817 and James b. abt 1819.
I am not aware of the surname being spelled MacTavish in any of the New Brunswick records I have searched, and I have not found him referred to as John Sinclair MacTavish, but please contact me if you think this is the same family you are descended from.