Thanks for that Carole.
I have found two possible Margarets aged 10 on the 1841 including the one you found. One I think it is in 1841 is at 13 Rose Street Piece SCT1841/98 with
Alex 54
Catherine 38
Alex 14
Isabella 12
Margaret 10
The reason I think it could be this family is that Margaret married in Portland Victoria in 1854 and on her marriage cert has these as parents and her father's occupation as Merchant. This Alex is on the 1841 as a Coppersmith.
But it could also be this family you have in 1851 but I have not found a marriage for this Catherine which would have to be around 1826 or earlier based on Alex being 15.
I am also not sure if the Inverness christening records are available to check out the father for this Margaret with mother as a seamstress in 1841 and then remarrying as you have found on the 1851 census.
I also found a possible arrival for Margaret in to Victoria in 1853, but I am not sure if the whole family went there or only Margaret.
Thanks