The Census of 1931 isn't available to the public until 2023 (https://en.wikipedia.org/wiki/1931_Canadian_Census), but let me see what I can find out through the Toronto Public Library and the Toronto voters lists.
The Globe and Mail article about his funeral has this to say: "His father Colonel Arbuthnot was killed in the Afghan campaign. Three of his brothers were killed in the South African War, and another brother during the Great War. His uncle Sir Robert Arbuthnot, went down with his ship at Jutland."
The only Arbuthnot in the Canadian Virtual War Memorial who died during WWI is Neil Arbuthnot, son of Alexander and Isabella Arbuthnot of Collingwood, Ontario.
The first "sister" is in fact a sister-in-law. Sarah Luena Johnston and Oscar A. McNichol are mentioned in their daughter's birth record."Ontario Births, 1869-1912," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:QV9H-DS3T : 16 July 2017), Sarah Luena Johnston in entry for Verna Luena Mcnichol, 31 May 1907; citing Birth, Toronto, York, Ontario, Canada, citing Archives of Ontario, Toronto; FHL microfilm 2,381,775.
The FindAGrave entries for Sarah and her siblings are here.