Courtesy of the imuseum I found another marriage for 'my' Margaret Caine.

24 Jan 1867 she married John Cannell at Braddan.
In 1871 she was living with daughter Catharine Margaret Kewley age 11 and husband John Cannell at Abbeylands.
She was married as Margaret Kewley described as a widow and father's name was Thomas Cain.

So this proves that the John Kewley I am seeking has died before 1867 but I still can't find any trace of him after his marriage to Margaret Thompson nee Cain in 1859.
I still don't know if she was the Margt Kewley wetnurse at Gravesend in 1861. If that was her - where was her daughter?
However it does prove that their daughter was still alive in 1871.

Just thought Donna and Jean might be interested and of course if there are any other suggestions I'd be grateful.