I have a LOT of this Sinclair family information for you! I have John's birth year as either 1782 or 1786, depending on the census. Sarah,b. 4June,1788. She was the daughter of William Jamieson and Betty Sweet. John and Sarah (aka Marion) had the following children:
1. Archibald, Jan. 20,1813, Killinian (m. Elizabeth MacIntyre in Ontario)
2. Alice, 8 Feb.1814, Killinian
3. Duncan, 4 Nov. 1816, Bowmore
4. Alexander, 17Oct. 1820 Bowmore
5. John,29 July, 1822, Upper Killinian
6. Donald, 9May, 1824, Bowmore
7. Robert, 20 Aug. 1827, Upper Killinian
8. Catherine, 18 Dec. 1829, Upper Killinian
I believe Archibald emigrated first. The rest of the family were still on Islay for the 1841 census - but in Kilchoman parish. - John55, Marion 50, Elizabeth 25, Alexander 20, John 15, Donald 15, Robert 13, Catherine 10. Plus 4 MacDearmid children: Ann 6, Marion 4, Duncan2, and Mary 1month. I think they were Elizabeth's children. And I also think that the name of Alice and Elizabeth were interchangeable.
I have found a couple of books with some info written about this family. By all accounts the sons were big men and very musical. All six of the sons took up land in Minto township, where they all farmed, except for John, who moved to neighbouring Erin Township where his wife's MacLean family also settled. A very good friend of mine is John's descendant, and he is also a strapping big and musical man. Which of the brothers is your ancestor? I can give you a lot of info about any one of them, but perhaps we can leave that discussion between us, unless there are others who are interested in this particular family.
Now, back to John's father - The Sinclairs on Islay (and much of Argyllshire) were known by the name of McNokaird (various spellings) before approx. 1750. But they predate parish birth and marriage records, so I'm very interested to learn the source of that information which says his father was born in March of 1740. Did Geni give his first name? I think I have the names of all the McNokairds on Islay from old tenant lists, etc. and the names of the farms where they lived. I'd be very interested to know John's father's name so that I can finally bridge the gap.
I hope some of this is helpful.