Your question identifies the way in which Maria Teresa differs from her other sisters. Perhaps, as befits her position as the baby of the family, her whimsical nature is part of her identity as the youngest sister. It is easy to distinguish her accounts from those of her more serious sisters, whether it be the self-righteous accounts of Minerva focusing on justice or the religious based writings of Patria. You might like to focus on her diary entries and the way that she uses a high number of exclamatory phrases and reflects her changable nature through her inability to stick to writing regular diary entries:
I have been a disaster diary keeper. Last year, only one entry, and the year is already half over and I haven't jotted down a single word. I did thumb through my old diary book, and I must say, it does all seem very silly with all the diary dears and the so secretive initials no one would be able to decipher in a million years!
Note the way in which Maria Teresa makes fun herself of her rather innocent, girlish way of writing. Alvarez uses such techniques to create a distinctive voice of a girlish, young woman who nevertheless is forced to grow up through the tyranny that surrounds her life and impacts on those around her. Her whimsical nature is shown through the way that so much of her account is based on those men she is attracted to and the various dreams and fears that she reveals.