While trying to tidy my study I found an old letter on Kerr research my aunt did which might help you.
I don't know where all her info came from but I know she was writing to cousins in the UK.
James Ker b. c1718 at Cleughbrae, d. 3 Dec 1789 age 71 buried Cleughbrae.
He married
Martha Moffat b. c1727 died 4 April 1802 buried Cleughbrae
they had 7 children and three of their sons James George and John married three Tweedie girls.
John Ker M. Jean Tweedie b. c1757
George Ker m. Mary Tweedie b. c1755
James Ker m Janet Tweedie b c1771
she thought that Mary and Jean were sisters and that Janet may have been a sister or niece.
James Ker and Janet Tweedie appear to only have had the one child who is my ggg grandfather
Thomas Kerr. His first wife was Mary Hutchison who died before him and was the mother of his 8 children and Agnes Forsyth who survived him and died in 1895.
I thought I might get an extremely large piece of paper, print out all the pages of this post and see if I can map the Kerrs from the information. Should keep me amused for a week or two