In the Scotland census 1851 Jean is staying with her mother Ellen (Helen) Weir but transcribed as Jane Blaw! James Blair aged 3 is with his mother and grandmother.
According to the two separate transcriptions I am viewing Ellen Weir, 63, was born in Kilmorich, Argyll; her daughter is Jane Blair, 24, and her grandson is James Blair, 3. These agree with the index listings at
www.scotlandspeople.gov.uk1861 Scotland census Helen referred to as Helen Wirs aged 73 mother! of James Blair instead of grandmother correct birth place for Helen is Lochgoilhead on the census
On both the transcriptions I am looking at, she is listed as Helen Weir, 73. This matches the index at Scotland's People.
Also she is recorded as mother of
Jane, who is the head of the household, not of James, who is correctly listed as son of the head of the household, that is, the son of Jane.
Evidently you are relying on some dud rubbish transcription (Ancestry?).
There is no substitute for viewing the original documents at Scotland's People.
Lochgoilhead and Kilmorich is a parish in Argyll. Kilmorich is near the head of Loch Fyne.
See
https://www.geograph.org.uk/gridref/NN1810