A suggestion to your inquiry could be to focus on Bernarda's obsession with keeping her daughters chaste, virginal, and free from sin. After all, she is now the head of the household, and the primary protector of every female in her family.
When the play begins we find Bernarda, ever more domineering and insensible, imposing upon their daughters the traditional eight year mourning period. This entails that the daughters who are not already engaged will have to age and waste away all due to tradition.
However, the promise that Bernarda makes to her late husband is that she will carry on the responsibility that the father often takes: to protect and preserve the honor of the females of the family.
Your monologue will then put Bernarda right at her husband's wake or during his burial. Make the monologue into a series of promises on behalf of the daughters. She can promise that Adela will embody virtue by waiting until the age of (28? 30?) to be seen out in public with her future husband, in order to preserve her dignity. Remember that Adela, being the youngest daughter, is the more likely to defy the mother. Hence special emphasis must be placed in promising to be in complete control of her.
Bernarda should also promise that Angustias will carry on with her marriage to Pepe, and that Martirio will show more care of her sister. Hint at the fact that Martirio's own feelings for Pepe will not disturb the peace and self-respect of the family. Also, she can also promise to take care of Maria Josefa, her own mother.
Considering that all of these promises may not be carried out, and that the end of the family will be tragic, it would be interesting to add your monologue as a prologue within the play, or add it to the first scene as the introductory scene.