Hi, Linda, just saw your response. Eliza Ann Morrison was Sylvester's 2nd wife. The 1st wife, whose name has not been discovered, died (perhaps in Boston) after the birth of their second child Georgianna Bates in 1843. Sylvester married Eliza Ann Morrison around 1847. She is not the mother of the children.
Sylvester and Eliza are not buried in unmarked graves. I have seen their gravestones with their initials, in Malcom, Iowa.
BTW my family are the descendants with the Civil War diaries of son Norman Francis Bates and I have the transcripts. He was awarded the Medal of Honor although that has unfortunately been lost by cousins.
As always, I'm very interested in hearing from people who are following this family!