Hi RJ and Annie
RJ, I think you mentioned you have subs to A/try? You can do a search on the 1891 and 1901 censuses, for example, for a Donald M*cK*n*n (to pick up variants) and then add the siblings you have to see what shows. I can see a couple of families with those children's names but not in the area. Without knowing the birth year for father Donald, hard to know whether Donald would still be with family by 1901, or away working by then. An imprecise science!
The son Robert you are looking for may not have been born in Invernesshire area. There are 4 entries showing for a Robert's birth in 1919 (assuming again that the year is correct), somewhere in Scotland. You can't view any birth cert images online past 1913 (100 yr privacy rule always), so anything after 1913 currently would need to be ordered.
For Robert Donald McKinnon, you have a birth day of 19 May 1919 (from his death reg), father Donald, a fisherman (from his marriage). However, birth details may not be correct. If the family fail to find something, you could as the GROS to do a specific search for his birth with this info, or close to. Cost a little bit more... Guessing everyone is down in England and not able to search in person at one of the main genealogy centres in Scotland.
I don't know what the archives are in Skye for fishing vessels, or what of records were kept for them
Maybe something you could check given you have at least one boat's name ('The Swordfish').
Monica
PS: One thing to note when you have fishermen, they may also show as crofters. Often they combined both occupations, to work both the land and the sea as a way of living.