The NAB gives this commentary on Mark 6:3
The brother of James . . . Simon: in Semitic usage, the terms “brother,” “sister” are applied not only to children of the same parents, but to nephews, nieces, cousins, half-brothers, and half-sisters; cf Genesis 14:16; 29:15; Lev 10:4. While one cannot suppose that the meaning of a Greek word should be sought in the first place from Semitic usage, the Septuagint often translates the Hebrew ah by the Greek word adelphos, “brother,” as in the cited passages, a fact that may argue for a similar breadth of meaning in some New Testament passages. For instance, there is no doubt that in v 17, “brother” is used of Philip, who was actually the half-brother of Herod Antipas.
It seems to be fairly common in many languages that “brother” simply means a male relative usually in the same age group. Often there was a way to make it more specific if it referred to a brother who was born of the same parents.