They don't buy a ranch. Not at first, at least, because their mother wants to stay here in the house their father paid for.
What does happen is that Biff gets a job as a clerk.
"Not for long," he tells Happy, "but we've got to take care of Mom." It isn't for long. Linda finds out a rich elderly cousin was recently widowed, and after a few correspondences Cousin Anne moves in, willing to pay for the care Linda can provide her with. They can go now, Linda says firmly.
It's better this way, Biff supposes. She doesn't like to look at them much. So they take the $20,000 she gives them and they take a train West a week after Cousin Anne looks around the kitchen and smiles in approval.
Biff takes Happy's arm as they pass a pretty girl and says, "Don't, Harold," pulling him into an empty compartment.
"What?" Happy says, and Biff yanks the curtains shut and kisses him for the first time in years as they sway with the motion of a train headed West, headed away from the city where they killed their father (who killed himself).
Happy kisses back.