I am descended from a David DUTHIE born in Montrose in 1825. He had a brother William born ca 1838 in Montrose. This could be the Father-in-law of your Mary.
My ancestor (Great Geat Grandfather) came to South Australia in 1854. I have been searching for descendants of his siblings but had no success as yet.
Will be glad to correspond further if you think there may be a link.

Max Duthie
Ballarat,
Victoria, Australia.