Bobby? I'm sure the Twists NEVER met him! They know he exists, but I'm sure Jack made it known that Old L.D. Newsome had a college fund all set up for Bobby. Besides, look how Bobby was raised. You think he would want to leave a nice home and job/college to go up to Lightning Flat? I don't think so, an neither would the Twists. I could definately see Ma Twist leaving the Ranch to Ennis.
As for Ennis being isolated out there? How is that any different from his trailer?? At least he would out on the ranch doing what he likes best. And who knows, maybe he'll meet a handsome young buck at the feed store who needs a job as a Ranch hand?