what happened to the sisters' parents?

Antigone and Ismene's father, Oedipus, was disgraced after forfeiting his throne, gouging out his eyes and then dying in exile. Their mother, Jocasta, killed herself when she learned she had married and had children by her son (Oedipus). Ismene does not add any detail to the mythical background but rather suggests that she and Antigone are burdened by this past. She uses it as an argument to get Antigone to submit to Creon's demands.