The use of the moon symbolically seems to indicate the level of violence at each stage in this incredible novel. The moon seems to act as some kind of gauge of the level of safety or danger of the girls and the kind of repressed violence that there is at various points in the novel. Note the following two examples.
Firstly, in Dede's flashback in Chapter One, the clear moonlight is symbolic of the time of peace and family unity experienced by the sisters with their parents at the beginning:
She remembers a clear moonlit night before the future began. they are sitting in the cool darkness under the anacahuita tree in the front yard, in the rockers, telling stories, drinking guanabana juice.
Note how the "clear moonlight" seems to symbolically represent the openness and happiness of the family, before the "future began."
However, if we consider how the moon is referred to in Chapter Six, there is a marked difference as the description of the moon is used to foreshadow the slap that Minerva receives from her father and the trouble between them:
The moon was a thin, bright machete cutting its way through patches of clouds. By its sharp light I could see my father stop and turn to face me.
Note how the metaphor employed in this quote compares the moon to a "machete," and its "sharp" light effectively foreshadows the sudden slap that Minerva receives.
Thus it can be seen from these two examples that the moon is used to indicate violence or peace and the various states that the sisters experience in their lives.