It's just speculation on my part but, I wonder if Jack Twist was adopted as a baby by Mr. and Mrs. John Twist? Jack never mentioned this in the dialog, but there is something about the tale of his younger life that makes me wonder if he was adopted. Perhaps the "uncle" Aguirre mentioned who had pneumonia is his biological father? Pure speculation on my part.
Note the scene in the Twist kitchen when Ennis comes to ask about the disposition of Jack's ashes. "He (Ennis) couldn't see much of Jack in either of them...." Maybe something,that for country ranch people of that time, happened that was "shameful" and Jack was the product. The baby Jack ended up with the Twists. That could partially explain why OMT never warmed to his son, "He kept his secrets to himself. Never taught me a thing. Never once come and see me ride." OMT may have been compelled out of some obligation to adopt and raise Jack. But not being his biological father may have resented him in his house. any thoughts or observations on my speculation?