I note that on Marys birth record 05/09/1840 Kilmarnock she is recorded as the fourth child of Alexander McDonald. So there must have been another older sibling (who died) in addition to Hugh and Donald.
Do you have the McDonald-McLean marriage record to see if this was the first marriage for both parties?
MCDONALD ALEXANDER marriage to AGNES MCLEAN FR2642 (FR2642)
15/09/1835 597/ 110 163 Kilmarnock
Yes, there is that marriage, however that would come after the birth of Hugh (1830) & Donald (1832).
I can't find any definite birth records for them.
We have tried writing to Norma McLeod at
https://skye-roots.co.uk However she was also a bit baffled. She said someone else had written to her many years ago about this same person, Mary McDonald. I think pre 1840, records on Skye become patchy. Note that on the 1851 census everyone gives their place of birth as Skye (apart from Alexander who gives us, surprisingly "Ireland", which may or may not be correct). 1841 census only gives info if the person was born in the county or not. In 1841 census all but Mary were born outside of Ayrshire.
Also, on the marriage cert, Mary gives her place of birth as "Skye", however, if Kilmarnock is correct, maybe her parents told her "Skye" because they were proud of their heritage.