Tía Mimi is, for a long time, Yoyo's maiden aunt. That means she isn't married, even though she's hit the ripe old age of twenty-eight and is probably doomed to be an "old maid" (3.2.8).
Mimi's not married because, according to the family, she's had too much education. Everyone knows Mimi is the genius of the family, because she likes to read books. She even got to go to college for two years (unlike the rest of the girls), just because she liked learning so much.
Mimi serves a twofold purpose in the novel. She's another illustration of the sexist attitudes of the de la Torre family about the role of women. Boo. But she's also a beacon of hope—the lone rebel in the legion of hair-and-nails aunts, setting an example for her nieces. Yay. She's a woman, and she does what she wants. Get it, Tia Mimi.
The family tree at the beginning of the novel lets us know that, to her family's great relief, Tía Mimi did get married, "finally." We hope it was to someone who appreciates how smart she is, and doesn't think learning is just for boys.