what is the significance of the Garcia girls nicknames? why, when she gets older, is Yolanda so opposed to her many nicknames?

Nicknames are a motif throughout the novel. They convey a sense of belonging and intimacy. The nicknames become terms of endearment like when the sisters call their mother "Mami". Yolanda doesn't like her nick name "Josephine" which has angelic connotations. She doesn't like this stereotype forming her identity.