Sue,
I don't know which Margaret Kissack she might be.
I see both of the Kissack lines in Seed of Isaac but unfortunately don't see that there are wills of the two fathers, Isaac and John.

The MI for Isaac is "terrific"
Lezayre Old Yard
#582
Isaac KISSACK

Burial Register
Isaac Kissack 26 Feb 1838
John Kissack 18 Nov 1836`

The death age of the Mg Lewaigue buried 23 Sep 1870 certainly fits either Margaret and is a much better match than the one o fPt-y-Vull age 64 who was buried 16 Apr 1860.
I was struck, I think by the residence of her and the 2 Marks.