Hi Cathy, I agree with you this IS a mysterious family. I did find that Robert C. Fannin Born: Illinois Residence: Healdsburg, Sonoma, CA Occupation: Wheelwright Enlisted: 1 July 1863: in the Civil War Marital Status: Single [interesting but maybe just a mistake] Previous Service: Mexican War under Fremont Source: U. S. Civil War Draft Registration Records 1863-1865.
I also found the Martin Family Tree, trees.ancestry.com/tree/20299022/person/927003968 which contains your Fannin family.Their daughter Harriet lived to adulthood, married and had a large family.
I didn't find anything else about Robert, Harriett or Isaac.
Death records weren't required state-wide in California until 1905 although some counties required them earlier.
There is a volunteer for lookups in Sonoma Co. at geneasearch.com/lookups/ca.htm . That's worth a try if the volunteer can do a local lookup for Robert and Sarah's death info.
I did look the three of them up in Findagrave.com but didn't find them.
Another idea is to contact the Sonoma County Genealogical Society, scgs.org/scgs.html. I'm sure they do research for a fee. Successful Searching!