Genealogy roadblock

I've hit a genealogy roadblock, one of those unanswerable questions, at least so far. It's not a direct descendant, a side branch. But, still it is frustrating. The photo above is of Mack Kerr and his son Mack, Jr. They are part of my roadblock.

My husband's great great grandfather, John Moon, had three sisters: Eda Jane, Lillie and Laura. Eda Jane and Lillie married brothers Mack and Elijah Kerr. My roadblock is finding the father of Mack and Elijah Kerr.

In the 1880 Sevier County, TN census, Mack is 6 years old and Elijah is 1 year old. They are living with their single mother, Nancy, age 21 and their widow grandmother Sallie Kear, age 54. Nancy is noted as a daughter of Sallie, not a daughter-in-law. The census notes Nancy as single, not divorced or widowed.

In the 1870 Sevier County, TN census there is a Daniel and Sallie Kerr with two daughters, Minerva, age 18, and Nancy, age 16. The ages are not 10 years different, but they seem to be the same people.

In December of 1880 Nancy Kerr marries William Gann and has other children.

I found WW I draft registration for both Mack and Elijah which gives their birth dates. However, neither one actually served in the war. My next step is to try and locate a birth or death record for Mack or Elijah which might note who their parents are. But so far, no luck.