What is the ruling on marrying the sister of a sister through رضاعة (nursing), and what is the definition of sisters through رضاعة (if the sister through رضاعة is the middle sister among eight sisters)?

Marrying the sister of one's sister by lineage is not forbidden, unless there is another reason for the prohibition. So, if you have a sister from your mother's side, and she has a sister from her father's side, then this sister from her father's side is not forbidden for you unless there is a prohibitive suckling relationship. The forbidden women are those included in the noble verse: "Forbidden to you are your mothers and your daughters..."