About 13 minutes before the end of the movie, when Young Matt leaves the house where Old Matt, Aunt Mollie and Sammy are tending to Pete after Pete was shot, Sammy follows, shutting and latching the door behind her. A few minutes later when Old Matt leaves, the door is standing wide open.
It would be quite impossible for Granny Becky (who has been blind her entire life) to be able to instantly recognize the familial similarities between Young Matt and Old Matt right after her eye bandages were removed.