Not sure whether this will help.
On Family Search there is a Agnes Renwick baptised 12 August 1781- Dumfries - maybe Peter's sister.
Also found Peter Renwick & Jannet Smith's four children, the earliest born 1812, which may give you an idea of the year they married.
Joseph Renwick bapt. 5 March 1812 Morton
Mary Renwick bapt. 15 Sept. 1822 Morton nr. Thornhill
Agnes Renwick bapt. 12 Sept. 1824 Morton nr. Thornhill
Janet Renwick bapt. 8 Feb. 1829 Morton nr. Thornhill
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