So before I begin this I will let you know there will be spoilers in this piece about Red Dead Redemption 2.
This is not the theory about how jack isn't Johns son, because in all honesty I don't believe in that. This here is a theory that I have not seen posted yet.
So at the ending of RDR we all know that John dies and his son Jack takes vengeance and kills Edgar Ross. During the days RDR first came out a lot of people were confused about how Jack could learn the gun slinging skills he had in order to take his vengeance since it was only him and his mother. Or was it only him and his mother?
Throughout RDR it is believed that the only remanding members of Dutch's old gang was Javier , Bill, Dutch, Abigail, jack, and John himself. However that is not the case and here we will enter spoiler territory for RDR2.
So at the end of the epilogue in RDR2 there are two people who survive and that were part of Dutch's gang that were not mentioned in RDR. Those two people are Sadie Adler and Charles Smith. My theory here however evolves around Sadie and the Marstons.
During the epilogue It is obivous that Sadie and John are close as Sadie acquires John help on hunting down bounties which eventaully leads to John asking Sadie if she wished to build a house on the property and that he would be more then willing to let her do that. Sadie is also tracking down Micah during this time.
So lets skip ahead now. John is dead. Jack and Abigail are trying to get as far from the Pinkerton as possible and Sadie is god knows where. I believe that Sadie would have heard news about Johns death and when she does she goes in search for Abigail and Jack to make sure they are ok and to get the full story and to protect them if needs be.
Sadie searches for abigail and Jack using all the resources and skills she has ( which is a lot ). After finding searching and finally finding them she gets the full story about what happened. She learns that one of her friends was gunned down by a pinkerton, a pinkerton that they had encountered before. Jacks upset and wants vengeance however his mother disaudes him from it because she doesnt want him to end up like John. Sadie knowing what it feels like to be robbed of a loved one and wanting to burn everything to the ground that the robber had starts training Jack with the skills she knows.
A few years pass and Abigail is dead. With his mother being gone and Jack having the skills he needs he decides to track down Edgar Ross and wins against him in a duel.
I believe that my theory could be true. Mainly because it covers the point a lot of people made about how Jack could have learned his gun slinging skills and how Jack and Abigail could survive on their own. Anyway its just a theory and I apologize about any grammatical errors I have made