I have been researching a family of Dewars who settled on Lot 29 Concession 7 in Lochiel. Your Isabel Dewar may have been a member of that family. The family was headed by John Bàn McLeor (Dewar) and his wife was Catherine McLeod. They arrived on the Argyle from Glenelg as part of the McLeod Settlement.
I have the names of her brothers and considerable information about them. The information I have differs in some details from the unsourced information presented by Keith Dewar in his books. As you may know… the family were called McLeor for many years after they arrived from Scotland and later adopted the anglicized version, Dewar.
Here is the source which ties an Isabel, born in Glengarry in 1810 to John McLeor and Catherine McLeod. http://www.glengarrycounty.com/standwmst.html
Cat # 2125 St. Andrews WMST B. M. D. the 2nd volume of St Andrews Presbyterian BMD Williamstown name index for the years 1805 - 1914 inclusive ISBN 978-0-921307-73-0 p. 67 Line 2 McLeor, Isabel Father: John McLeor, Mother: Catherine McLeod, Born: 14 Mar 1810, Baptised: 27 Mar 1810, Location: Lancaster, Item: B26, Reference St. Andrews Williamstown
If you are still searching for Isabel let me know.