Are the children of my maternal grandmother's husband, and of his wife who nursed me, considered my siblings, and is it permissible for me to marry the daughter of my maternal aunt, my mother's full sister?

Your maternal aunt's daughter is considered one of your mahrams if she was breastfed by your maternal grandfather's wife with a valid breastfeeding; this is because she would be the daughter of your sister through breastfeeding, due to the Prophet's (peace and blessings be upon him) saying: "What is prohibited by lineage is also prohibited by breastfeeding." Furthermore, all daughters of your grandfather, from whose milk you were breastfed, are considered your sisters through breastfeeding, whether they are his daughters from your grandmother or from another woman. Therefore, it is not permissible for you to marry their daughters, as they would be the daughters of your sisters through breastfeeding.