I think the easy answer is Minerva, who is the most vocal and political of the four sisters. But, personally, I don't feel she is the most heroic. She is the most revolutionary, and, in terms of the country's morale, she is certainly a hero, but within the context of the novel (which is fictionalized, you must remember) I think the hero is Maria Teresa.
Mate (her nickname) is the baby of the family. She is the most fearful, and the least interested in revolution (after Dede, who is prevented by her own fear and her husband from becoming involved with the underground). She is not brilliant, like Minerva, she is not a woman of great faith, like Patria; she is actually a rather frivolous person, who likes pretty dresses, and parties, and sweets. But in prison she shows perhaps the most fortitude of them all, and she suffers the greatest loss -- all without complaining. She drops a note for the OAS overseer (which she hides in her long braid) which eventually leads to the release of all the women prisoners she's been incarcerated with.
Mate's behavior when she was tortured (by electric shock, which made her lose her baby) was, perhaps, which made me admire her the most of the sisters. She didn't complain or pity herself, though she had good reason. After she had been tortured in front of her husband, she would not let the guards help her with her clothes, or help her walk. This is an example of the quiet defiance she displayed throughout the novel.
The contrast between her and Minerva's somewhat bombastic and slightly selfish revolutionary activities is that Mate performed her acts of heroism out of love. Minerva, while certainly a sincere revolutionary who had very pure motives, was the cause, it could be argued, of her sisters' deaths. Minerva didn't have to enlist the help of her sisters in her revolutionary cause. Though Minerva may have been instrumental in the eventual fall of Trujillo, she also indirectly caused the death of her sisters.
It depends, of course, on what you consider a hero. Minerva was certainly a hero of the revolution; Mate was a hero to her family, and to those around her.