Is it permissible to marry a girl who breastfed with my maternal uncle, knowing that my maternal uncle breastfed with the girl's mother?

Your maternal uncle's suckling from the mother of this girl does not make her daughter forbidden to you, as she is not your maternal aunt through suckling. The prohibition does not extend to the relatives of the suckled. If the girl did not suckle from your maternal grandmother, then it is permissible for you to marry her. However, if she did suckle from your grandmother, then she is your maternal aunt through suckling and is not permissible for you, due to the Prophet's (peace and blessings be upon him) saying: "What is forbidden by lineage is forbidden by suckling."