Unless you're lucky enough to find someone who has already done the work, it is basically a matter of idenfifying all the possible candidates and then weeding them out, one by one.

I notice there are a couple of Wills for a Thomas Cubbon in 1873, one in Arbory and one in Ramsey.
These might give some clues.
Wills of possible parents may help as they often mention grandchildren.

You might want to look at the Clugston side. Wills of her parents may also provide some clues.