Unfortunately, you need to remember that the book presents us with all four of the sisters and their own separate characters, identities and journeys to becoming rebels against Trujillo's regime. Therefore, although they become united as a cohesive group against dictatorship and become an important symbol in the rebel movement, they all join that movement at different stages in their lives, with Maria Teresa being one of the last. Clearly, Minerva is one of the first, however, there is no quote in the novel that gives you want you want. You need to think of how the sisters, in spite of the opposition that they face, remain steadfast to their ideals and beliefs. Picking out individual events like that should be able to help you find the evidence you need. But remember - part of the beauty of this book is that we are shown las mariposas as being different and separate from one another, rather than a homogeneous group.