If we read the book carefully we understand that, although the sisters were very important figures for the revolution against Trujillo's regime, at the same time it was only through their death that they were transformed into something more than that, and became real symbols of resistance against Trujillo, and symbols too of his cruelty and violence. If we have a look at the final chapter, which records Dede's memories of what happened after their slaughter, we can see the beginning of this transformation. Note the following quote and what it tells us about the sisters:
People came out of their houses. They had already heard the story we were to pretend to believe. The Jeep had gone off the cliff on a bad turn. But their faces knew the truth. Many of the men took off their hats, the women made the sign of the cross. They stood at the very edge of the road, and when the truck went by, they threw flowers into the bed. By the time we reached Conuco, you couldn't see the boxes for the wilting blossoms blanketing them.
Thus it is that the legend of the Mirabal sisters was created, and, ironically, they became a far more potent force for freedom through their deaths than they ever did in their lifetime, gaining a fame and reputation amongst all Dominicans that became a focal point of resistance.