Two general guidelines.
One, never, ever trust anything you find online (especially online family trees) unless it's an image of an original document - and even then, be wary, because mistakes can and do happen. You've noticed yourself that some people have got two George Shearers mixed up.
Two, don't try to read anything into spelling variations. Spelling wasn't standardised and it often depended on the individual clerk's idea of how a name should be spelled. The George Sherar married to Jan(n)et Bruce is spelled Sharer in some of his children's baptisms and Shearer in others.
The source of almost all original records is
www.scotlandspeople.gov.uk.
I see that William Christie and Margaret Sh*r*r had six children
Jean Howie, baptised 16 September 1821
Margaret, baptised 29 December 1822
Elizabeth Shearer, baptised 12 August 1824
Janet Taylor, baptised 29 June 1827
William Sherar, baptised 15 December 1829
George, baptised 4 January 1835
In 1841 the family were in the parish of Liff and Benvie
William Christie, 45; Margaret Christie, 45; Jean Christie, 15; Margaret Christie, 12; Elisibeth Christie, 10: William Christie, 8; George Christie, 5; all except Margaret senior born in Angus. Note that ages of people over 15 were supposed to be rounded down to the nearest 5 years, so both William and Margaret could have been anything from 45 to 49, and if these ages were accurate, they could have been born any time from 8 June 1791 to 7 June 1796 (because the census was taken on 7 June 1841). Jean would have been 19, so is correctly recorded as 15. Also that the child Margaret is too young to be the one baptised in 1822, who would have been 18; either the first one died and a younger sister, missing from the records, was given the same name, or there is an error in the census.
The 1851 census, again in Liff, lists William Christie, 56; wife Margaret Christie, 62; daughter Mary Christie, 26; and a lodger. Margaret's birthplace is given as Wick, Caithness. If her age is accurate , she would have been born between 31 March 1788 and 30 March 1789, because the census was taken on 30 March 1851. As these dates do not overlap with the dates deduced from her age in 1841, clearly at least one census is wrong. Also where did Mary come from? She's not in the 1841 census, there's no record of her baptism, and she is four years too old unless she is a misrecording of the 12-year-old Margaret in 1841, who would by now be 22.
So you have Margaret Shearer, who could have been born any time from 1788 to 1796 in Wick.
Noting that Margaret seems to have named her second son George, and that she apparently named two daughters Margaret, and that in spite of having at least four and possibly five or even six daughters she did not name any of them Isobel, I
speculate that she is more likely to have been the daughter of George Shearer and Margaret Miller, baptised 27 February 1790 in Wick.
What other information do you have about her that makes you think she was the one born in Bower to a mother named Isabell rather than in Wick? Or to either Janet Bruce or Isobel Smith, neither of whom seems to have had a recorded daughter named Margaret, or any children baptised in Wick?
Have you found her in the 1861 census?