according to the chronology, Marston's daughter may have been born between 1899 and the Roanoke Ridge incident, or after 1907 and the arrival of Edgar Ross in 1911, and in both cases she died of a fever (the second option makes more sense to me, since John is finally well off).

In my theory John' s daughter died because she was Born prematurely perhaps Born in a severe and weakened state and died a few hours later, i think John's daughter was named Rachel and he gave his horse the name Rachel to honor his deseased daughter

I prefer to think that she lived and died between 1899 - 1907. That would explain why she doesn't have a grave in Beecher's Hope.

Is it said that the daughter was Abigails child as well. I can’t really remember cause if not that could be an explanation. Like he had the daughter before Abigail had even joined the gang and his constant denial that jack was his child was because he felt like he failed at being a father because he’s first child died. I don’t know could be another explanation.