@ray: what you say is only true for Roman Catholic Christian theology, there are many other churches that even Paul alludes to and at least another New Testament worth of books that were rejected by the Nicene council in the West. Gnostic means proto-orthodox, so coming before the current orthodox.
We’re talking about a sect of Christianity that split before the Pauline letters were even written and travelled up to India and developed in their own independent version of Christianity surrounding ‘the twinship’ of Jesus.
As far as our Roman Catholic interpretation of it, they don’t seem to conflict as Thomas, Judas, Joses, James, Jude and Simon are named as being Jesus’ brothers.
This isn’t Gnosticism, Matthew, Mark, Luke and John lists Thomas ‘the twin’ on several occasions, so it was quite important to know he was a ‘twin’. The twin of who exactly and why make such a big deal out of his name, well if you don’t define it as being someone else’s twin, that leaves it to the main character in their story.
In Greek mythology twins caused ambivalent reactions and were believed to have ambivalent feelings for each other. Very often, they were viewed as the representatives of the dualistic nature of the universe. And doesn’t that represent ‘doubting Thomas’. Ambivalent about the resurrection of Jesus. And Jesus himself, dualistic, both divine and human at the same time which is what all Christians believe, that the duality of humans (the divine and the earthly form) can be unified in Christ.
There is a very similar story to the Jesus story in Greek mythology, where Zeus comes down and impregnates a woman after already been impregnated (we know Mary was engaged) by a human, resulting in a twin birth, one divine. It’s not hard to see the similarities.