Checked the LAC web site today and they have digitized and uploaded the service records for William Robb Dewar, 53902, 18th Battalion and John Taylor Dewar, 730016, 4th Battalion. These documents are in PDF format.
Click here for William Robb Dewar, 53902, 18th Battalion.
Click here for John Taylor Dewar, 730016 4th Battalion.
For assistance in interpreting the service records please review the excellent guide at the Regimental Rouge called Researching Military Records.
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History of the 18th Battalion CEF, "The Fighting Eighteenth" 
Can anyone tell me if they had any other siblings?
Jenna,
I will as my mother. She is the daughter of William Robb Dewar. I believe there was one other son.
Eric
Jenna,
I have checked with my mother and there was one other son, David, and three sisters: Margaret (Gordon) Dewar, Isabel (McCann) Dewar, and Jean Dewar. I checked the 1911 census and they do not show up in Ontario so I cannot confirm dates of birth or spelling. I hope this helps.
Eric