Magdalena is the second-oldest of Bernarda Alba's daughters, and therefore the oldest of the daughters by her second husband. In the action of the play, Magdalena is more of an observer than a participant. She also seems to be the most realistic and cynical of all of them.
All the members of the household are at each others' throats. Bernarda is hated by the daughters, by the maid La Poncia, and probably also by her own mother Maria Josefa, who has dementia. The center of the plot is that Angustias, the oldest and probably least attractive daughter, who has inherited the just-deceased stepfather's wealth, is set to marry the much younger and sought-after Pepe. Magdalena is the one who openly comments, "Why would Pepe want her?" Magdalena says it would make far more sense for Pepe to marry the youngest sister, Adela, or the middle one, Amelia. She knows Pepe is only after Angustias's money. Magdalena says this in front of the other sister Martirio, who is at this point secretly interested in Pepe as well. Of all the daughters, Magdalena is the one who says that she herself will never get married—such has been the iron will of the mother, Bernarda, in keeping them all under lock and key.
Magdalena is somewhat like a chorus, commenting on the action, though only at brief and pointed moments. She's correct about Pepe's being more likely to take an interest in Adela. This, tragically, is what happens, and when Bernarda and the others realize Adela and Pepe have been together in the hayloft, she grabs her shotgun and shoots at Pepe. Though he escapes, Adela believes him killed and hangs herself. Bernarda's final act of tyranny is to declare that everyone will be told Adela "died a virgin."