Adam Clarke on Matthew 12:46

His mother and his brethren - These are supposed to have been the cousins of our Lord, as the word brother is frequently used among the Hebrews in this sense. But there are others who believe Mary had other children beside our Lord and that these were literally his brothers, who are spoken of here. And, although it be possible that these were the sons of Mary, the wife of Cleopas or Alpheus, his mother's sister, called his relations, Mar 3:31; yet it is as likely that they were the children of Joseph and Mary, and brethren of our Lord, in the strictest sense of the word. See on Mat 13:55 (note).