Thanks for all of your info and suggestions. I went through the Memramcook, NB church record page by page for an entry on Mary Anne Anderson, but there is none. The last mention of her is in 1885 when their daughter, Stella, was born.
However, there is an entry in Mary Anne's brother's journal dated 1887 where he says he paid "Mary Anderson" a few dollars for washing. So she had to have passed away sometime between then and 1891. Also, in the 1900 US census, Mary Anne's mother lists 8 children, 7 of whom were still living, so she definitely was gone by then.