They are not just called "brothers" in those verses, they are called "his brothers" and they are even called "his brothers" in my Imprimatured Catholic Bible. Bishop Challoner translated it "his brothers" -- not his cousins -- but "his brothers". Is your Greek better than that of Bishop Challoner???
Of course if one has chosen “brothers” to signify a group of cousins, second cousins, and sundry relatives, then in a possessive case we get “his brothers”.