Who are the protagonist and antagonist in the story, "A Jury of Her Peers"?

There are two interpretations that respond to your question.  In one reading, the absent, imprisoned Mrs. Minnie Foster is the protagonist for the meaning and action  revolve principally around her:  it is her story that her neighbors, Mrs. Peters and Mrs. Hale tell. She functions beyond herself as a character; she represents the women who figure out her story, because in many ways her life is theirs—which is why they understand it. The antagonist would be her dead husband, for it is he and all the forces of society—a patriarchal power structure—that he represents that repressed her to such a degree that she murdered him.  It could also be argued, however, that Mrs. Hale and Mrs. Peters are the protagonists, heroically protecting Mrs. Foster by figuring out the details of the murder. Their husbands would then be the antagonists because they function in direct opposition to them. Although these men appear so  inept in their actions and ideas that they are little match for the women. in the story, they nevertheless represent a power structure against which the women must contend.