Cannot be certain that she is the correct one & that Agnes is her mother, but looks promising. It will take some investigating.
Why would she be "Mrs" if she wasn't married?
I would also suggest that DNA is the way to go in this situation. Nesta
On the 1911 census, there is a Agnes B Graham, age 13, listed as the daughter of a George Whitten & his wife Maggie (nee Graham), age 31. Could Agnes, age 68 in 1921, be her grandmother?
A Hugh Brown Graham married a Maggie Minto, 1897. Daughter, Agnes Brown Graham, born 1898. Hugh died 1901, MMN Brown.George Whitten married Maggie, 1903.