Hmmmm...that would prove interesting. I found a website that lists marriages in Upper East Ontario for that time period and there are four Dewars listed:
Sally m. Donald McDonald 1836
John m. Mary McDonald 1836
Ewan m. Catheron Cameron 1840
Isabella m. Miles Campbell 1838
They could well be siblings.
The website is http://www.ontariogenealogy.com/easternontariomarriages.html
Even though the parents won't be named, I used the email link provided on the webpage to send a query regarding the cost of the complete marriage record.