In the 1851 census Isabella is shown as head aged 56 living with daughters Margaret (25) and Isabella (22) at Breadalbank Terrace, Pultney.
How sure are you this is your Bell and her daughters? Wasn't you Bell's daughter Margaret younger than daughter Isabella?
You say the daughter Margaret (of your Bell) died 1859 - was she married?
There is the following Census:
1851 White House, WickParish 43, ED 4, Folio 0, Pg 20, HS 75
Head: Janet MANSON 82, unm, Pauper, b Canisbay
Dau: Isabella MANSON 56, unm.b Canisbay
Dau: Margaret MANSON 48, unm, b Canisbay
Son Alexander MANSON 45, unm, Fisherman, b Canisbay
Gransdon: Thomas FARMER 13, Scholar, b Wick
Gr-Son: Alexander SHEARER 9, b Wick
Gr-Son: John MANSON 18, unm, Cooper, b Canisbay
Could this not be Janet nee BANKS and her children? Daughter Isabella might be you Bell, (It wasn't uncommon for a Scottish widow to revert to maiden name and state her marital status to as 'unmarried; though so that does cause some issues :-) and Grandson John the Cooper, could be Isabella's son?
Same Family in 1861:
1861: Address: Louisburgh, WickParish 43, ED2, Folio 0, Page 37 , HS 193
Head: Alexander MANSON 60, unmarried, Fisherman, b Canisbay
Sister: Isabella MANSON 67, married, b Canisbay, Caithness
Sister: Margaret MANSON 63, married, b Canisbay, Caithness
Nephew: Thomas FARMER 20, unmarried, Cooper, b Wick
Niece: Isabella PARK 9, Scholar (Orphan) b Wick
Niece: Margaret PARK 8, Scholar (Orphan) b Wick
Niece: Charlotte PARK 6, Scholar (Orphan) b Wick
Niece: Jessie PARK 4, Orphan, b Wick
The nieces at least, could be Great-Neices: This appears to be their parents:
Hutchison PARK and Margaret MANSON, married 1849 Wick
They are in Louisburgh 1851, with :
Dau: Isabella is age 10 mnths.
Visitor: William SHEARER 20, Cooper.
He's a fisherman, b Glasgow.
His wife Margaret is age 20, born Canisbay - about 1831
Cheers
AMBLY