According to obituary, he is survived by his widow, son David, and daughter Mrs. Hiram Harrington.
"Canada Census, 1881," index, FamilySearch (https://familysearch.org/pal:/MM9.1.1/MVN7-V55 : accessed 16 Nov 2014), William Caseley in household of John Caseley, Lot 18, Prince, Prince Edward Island, Canada; citing p. 47; Library and Archives Canada film number C-13162, Library and Archives Canada, Ottawa, Ontario; FHL microfilm 1375798.
It may be possible to confirm family relationships with William by comparing test results with other carriers of his ancestors' Y-chromosome or mitochondrial DNA.
However, there are no known yDNA or mtDNA test-takers in his direct paternal or maternal line.
It is likely that these autosomal DNA test-takers will share DNA with William: