Is it permissible for a girl to marry her maternal aunt’s son if his older brother was suckled several times (the exact number is not remembered) by her grandmother along with her deceased maternal aunt, keeping in mind that it is the younger brother who proposed to her?

Your fiancé's brother suckling from your grandmother does not prevent your fiancé from marrying you, as long as he did not suckle from her or suckle with you from another woman. This is because, in this case, he has not become your suckling brother. As for his brother, he is your suckling brother because he has become like a maternal uncle to you, and suckling makes unlawful what birth makes unlawful.