Is it permissible for me to marry my maternal cousin whose mother did not nurse her, knowing that my mother fully nursed her younger sister?

If your mother breastfeeds one of her sisters, that sister becomes a sibling to all of your mother's children, and thus, what is unlawful for your blood sisters becomes unlawful for her, due to the Prophet's (peace be upon him) saying: "What is unlawful by lineage is unlawful by fosterage." As for the maternal uncles and aunts who were not breastfed by your mother, it is permissible to marry their sons and daughters.