Welcome to G2G forum Terry.
Genealogy can have some surprises.
Dr. Edwin McClain was not Forda's nephew. He was Forda's grandson and he was also your mother's cousin.
The parents of Dr. Edwin McClain were Fred E. McClain and Martha Stanton:
According to the FindAGrave record of his mother, Martha S. (Stanton) McClain was born on 16 May 1896.
Fred, Martha and Edwin are listed In the following 1930 Census sheet (Lines 59-61):
Martha's father was born in Missouri.
The family of your great grandparents Clinton and Forda is listed in the following Census sheets:
1910 Census (Lines 71-77)
Your grandfather Mark was 4 y/o, he had a sister named Martha 14 y/o. Your great grandfather Clinton was born in Missouri.
1900 Census (Lines 10-13):
Martha was born on May 1896.
The mother of Dr. Edwin McClain, Martha, was the sister of your grandfather Mark.