Trying to find my great-grandmother's birth parents

I'm doing a personal genealogy project, and I'm trying to find any information on my great-grandmother's birth family. The earliest record of her I can find is the 1891 Census of Manitoulin District, Ontario, which lists her as 10 years old and the adopted daughter of William Size and his wife, the older widow Ann Umphries. She was born around 1882-86, and may have been named "Edna Georgina" or "Edna Georgena" though she went by "Ena". Her name on all of her kids' marriage licenses appears as "Georgina Weatherall" or "Georgina Weatherhall". She appears on the 1901 Census of Algoma as "Ena G. Size" and on her 1911 and 1921 Census info as Georgina Size. How would I go about finding some record of this adoption and thereby information on her birth parents?