Thanks for the good lead! I was wondering if Sadie was a nickname, and it appears as if her given name was indeed Sarah. The marriage certificate you refer to is indeed one of her children. I didn't remember seeing there previously. I did as you suggested and searched the Canadian census records for a Sarah McDonald born in 1878 who had a father born in Scotland and a mother born in Ontario, and got two hits. One of those families appear to have moved to North Dakota around 1890, which matches everything about my Sarah McDonald.
I may have to request a death certificate or a long form marriage certificate as you suggested to confirm my findings, but it seems like this Sarah McDonald may be the daughter of a John McDonald and Margaret McPeak.
Thanks so much for your help,