Turns out, the story is far tamer than that. Here is what the book has to say on the topic:
"If the old servant woman decisively corrected every one of the girls' (Monica and her sisters) failures, she was also wise, loving and prudent in her teaching and advice. She was a fervent Christian, and she instilled in Monica and her sisters a Christian view of life, reverence for the prayers and customs of the Christian community, and love for the poor. The accounts contained in the Augustinian breviaries about Monica praying through the night, sneaking out of the house to do to church, and depriving herself of food in order to give it to the poor are legends. But given her good natural disposition, the work of grace, and her upbringing, it is likely that she made quick strides towards holiness even as a little girl.
As would eventually be the case for Augustine, Monica's conscience was so delicate that in her latter years she would recount to her son things that she considered shameful failings, although these may well produce a good-natured smile from the modern reader. For instance, Monica's parents had given her the job of drawing wine from the casks in the cellar and bringing it to the table, because they knew that she was a sober and virtuous girl. Forbidden fruit tends to exercise a unique fascination on the young. Why should she not be able to say taht she, too, had drunk wine? It was something reserved for her elders, which was precisely why trying some herself became so attractive.
Monica may have been virtuous, but she was still weak enough not to be able to resist this temptation. She began by barely touching her lips to the flask; then, sipping a little more each time, she got to the point of gulping down almost a whole glassful. The only witness to her actions was another servant who accompanied Monica to the cellar. Shortly afterward, a disagreement arose between the two for reasons we do not know: In the course of the argument, the servant hurled in Monica's face the epithet meribibula, meaning "wine-swiller." Instantly ashamed of herself, Monica acknowledged her fault. So great was her humiliation that she decided to quit her bad habit immediately."
St. Monica, ora pro nobis.