Because this novel is written in a way that uses all four sisters' voices as individual narrators of their part of the story, you could examine the difference in tone between the four sisters. The novel opens with Dede because the novel starts in the present day and we learn very quickly that she is the only surviving sister and is left to tell the story and remember what happened to the others. Her tone is sad and kind of weary, but also determined to keep the memory of her sisters alive for the future generations. You could look for evidence to support this in the first and last chapters which serve to bookend the novel in the present day.
Minerva is the "ring-leader" of the four sisters who influences their involvement in the revolution. Her tone is usually determined and fearless. You could look for evidence of this in chapters narrated by her such as the chapter where she stands up for herself with Trujillo at the party, the chapters where she is first involved in the revolution, and the chapters where she is in the prison with her sisters.
Patria is the most motherly figure and the most religious. You should look at the chapters, told by her, about the loss of her first child, the day she saw the young man who was killed, her story of their time in the prison, and her reaction to life at home under the surveillance by Trujillo's henchmen.
Maria-Terese is the youngest, and has the most childlike and innocent voice in the novel. This is, in part, because her part of the story is from the diary entries written in her youth, which is one place to look for evidence of her tone. You could also look at her chapters, especially when she first joins Minerva as part of the revolution (but mostly to be around her future husband), and her more matured voice as she suffers physical torture in the prison.