When did the Mirabel sisters get the name butterflies in In the Time of the Butterflies?

The first mention of any of the Mirabal sisters being
referred to as a butterfly comes in Chapter Seven, which is narrated by Maria Teresa.
She goes to stay with Minerva and Manolo, yet one night she is surprised to be awoken by
a knock at the door. A man arrived with a delivery for Manolo. This man asks Maria
Teresa if she is "Mariposa's little sister." Thus it is that Maria Teresa finds out
about the rebel involvment of her sister and her sister's husband in the rebel movement
that is working to topple Trujillo. As one by one the sisters join the rebel movement,
the same code name is used to refer to them, and thus the legend of the mariposas is
born and the novel is given its title.