The painting itself is great though.

It is unlikely that the term “brother” is being used figuratively or mystically because all Christians are Christ’s brothers in that sense, making it pointless to single out certain individuals for this description. Full brother is impossible, as Protestants also acknowledge, since Jesus was not the biological child of Joseph. Half-brother is ruled out by the fact that Mary remained a virgin. It is possible they were adoptive brothers, but there does not seem to be any evidence for this in the biblical or patristic record.

More plausibly, they were step-brothers: children of Joseph who were Jesus’ brothers by marriage. There is some evidence for this in the writings of early Christians. The earliest discussion of the matter that we have–in a document known as the Protoevangelium of James (c. A.D. 120)–states that Joseph was a widower who already had a family and thus was willing to become the guardian of a consecrated virgin. Though not inspired, the document was written within living memory of Mary, when Christ’s family was still well known, as other sources attest (e.g., second century historian Hegisippus). It may contain accurate traditions regarding the family structure.

…There is no Aramaic word for cousin, and there certainly is no evidence that there was one in first-century Aramaic.

Check out http://www.catholic.com/library/Bad_Aramaic_Made_Easy.asp