I think that Hannah Hemstock may have died around 1851. On the 1851 Ontario Census, William and Ann Hemstock are in Sullivan Twp Grey County with their daughters Hannah, Ann, Fanny, Elizabeth and Isabella. On the following page are more details about the family ie their marital status, kind of residence etc. Under the column 'Deaths during 1851' there is a '1' for the Hemstock residence and I think that this may be Hannah.
Also, there was discussion that Hannah may have married a man named Doolittle. I found that in Hannah's sister Ann's husband David McBain's family there may be Doolittles related through marriage. Lycurgus Doolittle married Jessie McBain in Orangeville, ON in 1889.