Ah ha! I have smoked out Thomas Robertson in the earlier censuses.
1841 at Allachrow, Knockando, Moray:
Robert Robertson, "30", farmer
Frances Robertson, "25"
Thomas Robertson, "15"
Helen Robertson, "15"
1851 at Allachrow, Knockando, Moray:
Robert Robertson, 40, farmer of 50 acres, born Grange
Thomas Robertson, 23, farmer's brother employing 1 man, born Aberlour
Grace Robertson, 26, farmer's sister, born Aberlour,
James Edwards, 11, farm labourer, born Boharm
These entries show up Robert for the first time. In 1861 he is married to a Helen - still at Allachrow. They also show the twin, Helen, in 1841, and the sisters, Frances and Grace.