Is it permissible for me to marry the sister of the girl who was breastfed with my sister by my mother?

It is permissible for you to marry the sister of one who suckled from your mother, as long as you did not suckle from her mother, nor did she suckle from your mother, and provided she is not otherwise prohibited to you. The girl who suckled from your mother is considered a sister to you and to all those whom your mother breastfed or gave birth to. As for her sisters who did not suckle from your mother, they remain non-mahram to you, and it is permissible for you to marry them, unless there is another reason for prohibition.