yes, they are a confusing bunch!
I see on familysearch.org John McG's birth is given as abt. 1819, Sleat, and his marriage to Ann Grant is recorded as Feb. 13, 1846, which would put John age 27.
Both entries are Patron submissions, as opposed to transcripts from the Sleat PR's.
John is the eldest boy on the 1841 census, at age 20, so it would appear that at least John, and Christy were the children of Angus McG and Margaret Campbell.
The two younger siblings - Donald (age 13 in 1841) and Margaret (age 11 in 1841) could have been the children of Helen McDonald if her and Angus McG were married in 1825?
I don't have anything further on Margaret, but I know that Donald MacG married Peggy Robertson, Jan 31 1860 at Sleat, again from familysearch.org, (a transcript from the civil register.) I haven't seen the certificate.
It just might give a history of the many wives of Angus McG on his death certificate. Do we know if he died in Scotland? Or did the family all go to Australia?