What is the ruling on my marriage to my maternal uncle's daughter if her older sister was suckled by my deceased maternal uncle's mother, which causes her family to prevent this marriage on the pretext that she has become like my maternal aunt?

The suckling of one of the siblings from a foreign woman does not spread the prohibition of marriage between his siblings and the woman from whom he suckled, nor her children. Rather, it is restricted to the suckling individual only. Therefore, the younger sister of the suckling individual is considered a

non-mahram, and it is permissible to marry her, as long as no suckling occurred between them.