What do the Mirabal sisters learn about their father in the opening chapters of In the Time of the Butterflies?

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:

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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.