As far as I can find, the only sibling was John Peter Gordon Thompson, born 1905 in Bowmanville, who died young. However, birth records available to the public end in 1913, so there may have been other siblings whose birth records we cannot yet access.
There is a picture of the headstone here:http://www.canadianheadstones.com/on/view.php?id=277458
You might try contacting Mount Pleasant Cemetery and see who owns the plot and if there is any contact info. Or, if you can get a date of death for any of them, I can check for an obituary.