Is it permissible to marry the son of the maternal aunt if his older brother is a foster brother to the questioner through suckling?

If your maternal cousin (son of your maternal aunt) was not suckled by your grandmother, or your mother, or a woman who suckled you, then he is considered a non-mahram, and it is permissible for you to marry him Islamically. His brother being suckled by your grandmother does not affect you; rather, it makes him (the brother) your maternal uncle through suckling. The prohibition due to suckling extends to the children of the suckled person, but it does not extend to their siblings. Thus, it is permissible for their siblings to marry their sisters through suckling. Based on this, there is no objection to the marriage mentioned.