Martha Shelby was the first wife of John Shelby. She is the mother of four of John Shelby's children. Martha raised their children while he was away at war, and appears to have been a good mother and wife. She died before the first series begins. Not much is known about her and the cause of her death is unexplained. But John took her passing heavily, and the children fell in disarray.
"God takes the best first,"- Polly Gray to John Shelby about his wife's death.
it's her death that causes John to start drinking heavily. Eventually pushes John to marry Esme, so she will take over the parenteral responsibilities and control their children.