Welcome to Rootschat

I think your 1st port of call is to get the original 1851 from
www.scotlandspeople.gov.ukThis will list ages, where each were born, what their occupations were & relationships as you haven't mentioned any?
I'm guessing Margaret, William & Kenneth were kids of Abigail on 1851 i.e.
How old was Donald when he married Catherine?
Have you found him in 1851?
You need to try & find them in 1841 when they were all more likely to be at home together.
The 1841 won't show the same info. but probably enough to group them as a family (hopefully)?
From the info. you have provided, I would imagine Donald to be born 2nd son?
You have in 1851;
Abigail (what was her age)? Widow & married to a
Cottar would fit with Donald being a "deceased
farmer" on son Donald's marriage
Kenneth 30
Margaret 27
(Could Donald belong here)?William 21
Annie