Is it permissible to marry a male cousin if his grandmother breastfed him along with the questioner's younger maternal uncle, and the questioner is the daughter of the older maternal uncle's maternal aunt?

If your grandmother nursed your male cousin with five known sucklings, then it is forbidden for you to marry him, because he becomes your paternal or maternal uncle through suckling. This is based on the Prophet’s (peace be upon him) saying: “What is forbidden by lineage is forbidden by suckling.” And it is forbidden for an uncle to marry his brother’s daughter.

However, if he did not suckle five times, then the preponderant opinion is that he is not forbidden to you. This is based on Aisha’s (may Allah be pleased with her) statement: “Among what was revealed in the Qur’an was: ‘Ten known sucklings make marriage forbidden.’ Then this was abrogated by ‘five known sucklings.’” This means that anything less than five sucklings does not make marriage forbidden.