Sisters Mabel, Sophia and Mary all married in Algoma District.
They were living in Thessalon.
They are there on 1901 census.
William died 17 Feb 1917 at Thessalon and Agnes died 19 Dec 1907 at Thessalon.
Perhaps their obituaries would name Sarah Ellen as a survivor or predeceased.
Sarah wasn't a witness on any of her sisters' marriages.
Didn't find her listed as mother on any child's birth registration.
There is a Sarah Gavin married to a John Crockford but it isn't your Sarah.