Is it permissible to marry the daughter of a maternal uncle if the grandmother breastfed the questioner and his maternal aunt many times, and the questioner's age is compatible with his maternal aunt's age?

If your maternal uncle is your brother through rada'a (nursing), then his daughter is considered your niece through rada'a, and it is not permissible for you to marry her. What is forbidden by lineage is also forbidden by rada'a.