This is a deliberate attempt to have the reader see all sides of the effect of Trujillo's regime on the sisters and their nation.
The narrative begins from Dede's point of view, the sole survivor of the murdered Mirabel sisters. Knowing the sisters are already dead from the beginning helps us focus on what happened rather than making it a "thriller," waiting for them to escape or not.
Dede's story is filled with regret. She must live with the consequences of her decisions not to become involved in the rebellion. It will take her many years to accept that it is she who can continue to tell their story.
Minerva's is the most aggressive of the sisters. Hearing her side of the story helps understand both her strengths in fighting the oppression, but also underscores her sometimes intolerance toward her sisters, who are not as motivated as she or have family obligations in the form of husbands and children which she does not.
Maria Teresa gives the reader the sense of what it would be like to be a little girl, secreting away details in her diary, concerned with all the normal things that consume a young girls' life: clothes, boys, family. Having her perspective really humanizes the novel.
Patria's story helps the reader empathize with a person whose faith has been shaken. The loss of her baby is one of the features that makes "Las Mariposas" more than just mythical women, but real women, who lead real lives, and experienced real pain.