What is the ruling regarding the remaining older and younger siblings, are they considered siblings, if the mother breastfed the maternal aunt, and the maternal grandmother breastfed the sister?

Whoever shares the suckling from the same milk with another person is his milk-sibling. The prohibition does not extend to the siblings of the suckled child. Thus, the "aunt" would be a sister only to the children of your mother, and the "sister" would be a sister only to the children of her grandmother (her maternal uncles). As for the rest of the children from the mother and grandmother, they are not siblings due to not sharing the milk.