As we know jack looks like javier when he's 19 but when jack is 11 he is as stubborn as 1899 john. Covered by strange man on youtube Abigail was a prostitute so she sleep with a lot of members of the gang. even john says he don't know if jack is he's son. A bit of a clue in RD1 while capturing javier alive a secret dialogue would play it's rare tho "did Abigail miss me" teasing john of sleeping with Abigail. but if you want more information check out strange man on Youtube. I can't link as i'm using a school laptop.