I think it's probable the bluebird earrings are a way of suggesting Alma, Jr.'s similarities to Jack as an influence in Ennis's life.
Next to Jack, she is the person Ennis cares most about and has the idealism of youth. Like Jack, she loves Ennis unconditionally and seems to understand him better than others. She tries to come live with Ennis but doesn't press the issue when he refuses. In the final scene, she receives the sacrifice that Ennis would have gladly made for Jack at that point if it were not now too late.
Ang Lee and the writers make the parallel even stronger by bookending the first and last scenes in the movie. In each, Ennis is alone outside a trailer when Jack/Junior drives up. They are the same age in their respective scenes, too.