Finding Birth Parents of a Deceased Grand-parent

I am attempting to determine the birth parents of a deceased Grand-mother for the purpose of completing a family tree. I have talked to Adoptions and Operational Services of Ontario and they have told me that requests can only be made by the adopted person and that if the adopted person is deceased then there is no possible way to make the request. Are there any alternative ways of making these inquiries. I do not have any information on the adoption agency used. Thanks