James Morrison and Katherine McFarland had two children baptised in New Cumnock
James, 23 August 1808
John, 28 October 1810
Then there is your George in 1813, and another child with father James Morrison and no mother listed: Alexander, 31 July 1816.
Why not take a look at those and see what they say about the parents' residence? If James, John and Alexander were also born at Rigg, then it would be reasonable to suppose they were all brothers.
Agnes14, what year was that marriage? I don't seem to be able to find it on SP.