The '51 census does not mention the mother, the '61 census has her as Mary "Mcleod". But by deduction, this would more than probably be as the '41 census states, Mary "Lap", born "foreign parts". The '61 census has her as 61 years old, which would tie in with her being 40 in the '41 census.
There is an additional son "Grigor" born in 1866, Inverness.
Isabella is missing in the 1881 census, but there is a death of an Isabella McLeod, nee Beaton, Inverness 1880. The age shown is confusing. It has her born c 1836 which would match her age of 35 in the '71 census, but in the '51 and '61 census, both Isaac and Isabella are listed as born in Inverness, c 1825 and c 1831 respectively.
The other data would match up.
Confused yet??
Regards
Malky.