Is it permissible to marry the granddaughter of my maternal aunt, knowing that I was breastfed by my aunt once along with her daughter?

A single suckle, even if it is a single drop that reaches the stomach within two years, is considered prohibitory by the Malikis and Hanafis. Therefore, your female cousin's daughter would be considered your foster sister according to them, and it is not permissible for you to marry her. However, the preponderant view, based on evidence, is that prohibition does not spread except with five known suckles.