I’ve recently been interested in the Christ myth debate, something which still seems to be a white hot issue. I’m more in the “mythicist” camp for various reasons, and thought I should offer my lay opinion on one piece of supposed “historicist” evidence: Jesus’s siblings.
Jesus was alleged to have many “brothers,” one of which, James the Just, can be confirmed as a historical person. As Carrier already pointed out, “brethren” was an honorary title given to all Christians with no real way to distinguish between literal biological brothers and metaphorical ones. This is so common that the Oxford dictionary actually defines Brethren as “Fellow Christians or members of a male religious order.”
Jesus’s Improbably Large Family
One piece of evidence that’s not looked at is just how big the literal family would have to be. In Mathew 13:55-56 it mentions the four “brothers” (James, Joseph, Simon and Judas) and unnamed “sisters.” Taken literally, this means that Jesus had, on minimum, six siblings.
How likely would this be in ancient Israel? According to a study by Zorn, the best estimates we have of family size is 4-5.5 people. This is actually not much larger than families today but for a different reason: very high infant mortality rates and maternal health dangers making it unlikely that mothers would go higher than the norm. Jesus’s alleged family of nine (Mary, Joseph and seven kids) would at the very least be close to double that.
Another estimate by Perdue puts the theoretical limit on nuclear family size at seven based on the available resources and home sizes. Put another way: Jesus’s “historical” family would have been (at least) larger than what was economically possible in ancient Israel.
This could only be possible if (1) family compounds were used for two generations (something we do not clearly know) and (2) Jesus’s family used a two-family compound for a single family. Again, this would be like if a single person today (in both senses of the word) owned a family home. Again, it’s not impossible but absurdly unlikely.
Of course, we don’t have evidence to definitively prove or rebuff this claim but it’s clear in which direction the evidence goes.