I am finding your post confusing too.
You have two different death dates for Isabella, and other information that doesn’t add up.
So I hope this is what you are looking for.
In the 1881 census you mention, Isabella Jack 80, is in the house of her son-in-law.
Isabella’s daughter Margaret JACK married Alexander McDONALD (blacksmith) in 1844 Inverness.
Margaret born 1821 Inverness, was the daughter of Donald JACK and Isabella MUNRO
Donald JACK married Isabella MUNRO in 1820 Inverness
Isabella JACK nee MUNRO died (age 85) in 1885 Inverness - her mother’s maiden name - FRASER
Possible baptismIsabella MUNRO bapt 30/03/1797 INVERNESS – PARENTS WILLIAM MUNRO & ISABEL FRASER
Census: 1841 InvernessHouse or Street Name: King Street
JACK Donald M 50 Builder Out of County
JACK Isabella F 40 Inverness-shire
JACK Willm M 15 Mason Inverness-shire
JACK James M 15 Mason Inverness-shire
JACK Margt F 15 Inverness-shire
JACK Catherine F 5 Inverness-shire
JACK Collin M 6m Inverness-shire
CLARK Margt F 15 Female Servant Inverness-shire
Census: 1851 InvernessHouse or Street Name: King Street
JACK Donald Head M M 61 Mason Ross and Cromarty Knockbain
JACK Isabella Wife M F 51 Inverness-shire Inverness
JACK Colin Son U M 10 Scholar Inverness-shire Inverness