Are you refering to the discovery that they make regarding their father's infidelity and the way that they have a number of half sisters? If so, this is a discovery that Minerva makes in Chapter Six of this excellent novel, as she notices four girls who always come out whenever she drives the family car past their house. Note how Minerva describes them:
I studied them. There were three that ran to the road whenever they heard the car, a fourth one sometimes came in the arms of the oldest. Four girls, I checked, three in panties, and the baby naked. One time, I stopped at the side of the road and stared at their Mirabal eyes. "Who is your father?" I asked point blank.
Minerva thus discovers the existence of her half-sisters and also the fact of her father's infidelity. What upsets her most of all, of course, is the way that her father has not provided for his other children, not even making sure that they receive an education. It is important to note that this discovery also comes at a time when Minerva is facing her own struggles of wanting to go to university and to leave home.