Having lost her son, Jack's mother welcomes a son who has lost his mother and father. She loves Jack, Jack loved Ennis and Ennis Jack. so that's another circle she could complete, if she took Ennis, her only contact with both her son and with her emotions, and together they commemorated Jack. The shirts she must recognise as having had a significance to Jack that she doesn't entirely understand; I like to think that once Ennis arrives she makes the connection and gives him the shirts because they are his link, not hers, with Jack, and because she knows he'll treat them like a sort of relic - they hold the blood of her son, shed by the one he loved, all that was left after the rest was spilled by his killers.
As Ennis leaves she turns her back on John Twist, as a symbol, I think, of her rejection of him and his lack of love, and looks after the departing Ennis, who is now her only link with the past and the future.