Ooooo, good catch!
So here's another detail... very different context. I love how many empty chairs there are in the Lightning Flat scene. They're everywhere... there's one (or two?) behind Ennis and one behind John Twist and then one at the table and then one in Jack's room. These I'd say are meant to be constant reminders of Jack's absence. The number of chairs is striking since the house is otherwise so empty. Also, that black hat on the wall surely is meant to remind us of Jack (even if it really is John's hat or something).
Jeez! It's amazing how much details people are detecting. I noticed neiter the relatively many chairs nor the hat behind Ennis.
Once more I checked the screencaps at stripedwall.com (great source). I counted 4 empty chairs in the kitchen: one behind John Twist (at least this one I had noticed before), one on the right behind Ennis and two at the table between them.
I think the empty chairs are there to emphasize the emptyness of the Twist house in general and the absence of Jack. I don't think the Twists often have visitors; friends or family to come around and having a good time together (ha ha, old Twist and having a good time - an antagonism itself). I think these two people barely ever need six chairs in their kitchen. Old Twist has run off every friend they might have had a long time ago, I guess. And Mrs. Twist suffers from this. Her house, her
home is an empty and lonesome place. And it gets even worse after Jack's death. For Jack's mom, his visits must have been the brightest days in her year.