The man in the yellow suit has followed Tuck's family since they "kidnapped" Winnie because he suspects their secret. He convinces the sheriff of Treegap to ride with him out to Tuck and Mae's house to "rescue" Winnie. He arrives before the sheriff and tries to bribe them into revealing the location of the water that gives immortality to whoever drinks it.
They refuse. The man in the yellow suit then threatens to give the water to Winnie, whereupon Mae shoots him, just as the sheriff arrives. The sheriff arrests Mae and tells her that if the man she shot dies, she will be hanged for murder. This would destroy everything, of course, because Mae cannot die, and the secret would be revealed to all. Winnie and Tuck's family then devise a plan to help Mae escape from jail.
Winnie and the Tuck men broke Mae out of jail and the family ran!
Mae killed the man in the yellow suit and that is why she was in jail.
Winnie did NOT go with the Tucks and she lived to be 99. She married someone else and had children, lived a happy life and died a natural death.