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According to the 1891 census, there are two sons called Donald living in the household at the time, born 1879 and 1882 respectively. This is the elder Donald Macdonald, and from his birthdate he may have been a twin to Angus. However, as he doesn't appear on the census in 1881 or 1901, he was perhaps a visiting cousin, rather than a son.
The younger Donald is certainly a son of the family.