Lyn,
Yes I would use Scotland's People, it is home of the original OPR's.
I did take a peek and see there is a Margaret who married a Robert Tweedie in 17 Jul 1768 but no Mary.
And, although it states that both Margaret and Robert were of this Parish it doesn't mean they were born there.
However, if this Margaret was from the Parish and baptised then this is the only one on record.
SOMMERVELL MARGARET ANDREW SOMMERVELL/ 20/02/1732 (no mother identified)
Or, if this Margaret was baptised a Mary then the following the only one who would fit.
SOMERVEL MARY JOHN SOMERVEL/JANET MEIKLE 11/08/1750
Don