When we see the word “brother” (or “sister”) in the Scriptures, we need to be careful. “Brother” can, of course, mean a naturally born brother–having the same father and mother.
Or it can mean (natural) half-brother, or step-brother–having the same mother or father, but not both.” For example, in Matthew 13:55, which reads, “Isn’t this the carpenter’s son? Isn’t his mother’s name Mary, and aren’t his brothers James, Joseph, Simon and Judas?” In this case, Jesus was a half-brother to James, Joseph, Simon, and Judas, because Joseph was the father of Jesus’ “brothers” but Joseph wasn’t Jesus’s father (even though Mary was the mother of all of these sons).
We’re not sure, but “brother” may also be used sometimes in the scriptures as meaning “of the same clan, or tribe”–like a cousin.
However, brother can also mean a “brother” in the faith, in the church, or in the assembly.
So, we need to be careful about jumping to conclusions. We can’t assume that brother means a naturally born brother–having the same father and mother. So, did Moses and Aaron have the same father and mother? In Exodus 4:14 says, God asks Moses, “What about your brother, Aaron the Levite?” If they were natural brothers, then why did He have to qualify his identification with “the Levite”? We can’t be sure.
Jude 1:1 says, “Jude, a servant of Jesus Christ and a brother of James …” So were Jude and James natural brothers? Probably so, but again we can’t be certain. If so, there are apparently more than one (different) James referenced in the Scriptures, and we still can’t be certain which one it is.