Hi Karytay, that sounds about right - same year as her brother Patrick was married. Father, Adam Colquhoun was Keeper of the Register of Sasines for Dumbartonshire, died around 1753. Mother was called Isabel Colquhoun.
Both Isabella and Mary Ann are mentioned in Patrick's will, as is their brother John Lamont. A codicil to the will dated April 1820 refers to Mary (Patrick's sister) as living in Greenock. Patrick left money for her in case her son John died before her and she would become destitute. No mention of what happened to John Lamont Sr.
I cannot find baptisms for Isabella or John Jr either, but I'll keep looking and let you know if I find anything.