For the benefit of future readers, I had asked this question in a new thread, apart from my other query, to discuss the generic issue of adoption.
As you have pointed out, adoption in Ontario was legislated in 1921. A great explanation appears on the Ontario GenWeb site: Adoption in Ontario: A Brief History (http://www.rootsweb.ancestry.com/~canon/research-topic-birth...
Prior to 1921 I suppose there was a number of arrangements that could be made for the "adoption" of a child.
As far as Ada was concerned, when she appeared on the 1901 census as a 20-year-old "adopted daughter" I think that means she found a good family.