The “apocryphal” Gospel of Philip says Jesus had two Marys in his life – one the mother and the other his sister/lover. I give this Gospel some credibility because it was suppressed. My theory being that this “gospel” was suppressed because it alluded to the truth. In addition, the author Philip (a Herodian?) seems to be a relatively sensible person.
To make what appears to be a quantum leap – the first born by Judaean tradition was the property of God such as “The first born is mine”. I guess the first born was either sacrificed or disentitled through a substitute sacrifice of a new born lamb for example.
Why this tradition? – probably the first born (of royalty) was a “royal blood” princess mating with a commoner or stranger to prove that the royal princess was fertile. Having proved her fertility she could then be married off for mostly political reasons.
So Jesus in this scenario is the first born son of Mary the mother perceived as a virgin (she could well have been called “the virgin Mary” prior to the mating). And this first born is God's property; hence he becomes 'the son of God”.
All of this is probably the way it could have been viewed by the average Judaean of the time.
Who was the father? The mysterious Antipas seems highly likely and here's another possibility - rumor has it that the father was a soldier and I could speculate that Tiberius could have been in the area at the time since he was a apparently a family friend and very peculiarly sexually active especially in his later years as Roman Emperor.
Continuing my line of thought - Mary the mother having proved her fertility then went on to produce more offspring (Mark 6:3) with one or more husbands. This could make Mary II the half sister of Jesus - Mary the mother gave birth to Mary II.
These are the wanderings of my fertile mind and would require the rewriting of nearly all the history books so please regard them as mere speculations. In other words don't be too harsh with me for speculating in this way.
Meanwhile the Herod family tree has so many twisted branches that I don't know who to believe, so I am placing the family tree mostly on the backburner. It would take me months to sort it out!
© Charles N. Pope, US Library of Congress. All rights reserved.