Esteban's mother and sister are pretty much wretchedly miserable individuals. They're both characterized by a rigid morality that's associated with the Catholic Church. Doña Ester Trueba reads things like The Lives of the Saints while she festers in her bed and slowly dies. Férula likes to pray in the streets of the poor districts of town while its inhabitants dump garbage on her head. Talk about a martyr complex. These two women serve to illustrate the poverty and unhappiness of Esteban's childhood, and go a long way in explaining his sour character. Férula also acts as a foil for Esteban – check out our explanation of this under "Character Roles."
We should definitely note that Férula gets a lot more pleasant when she meets Clara and goes to live in the big house on the corner with the newly wedded couple. Whether Férula's crush on Clara merits Esteban's accusation of lesbianism is a matter of debate.