I always thought Jesus was a random follower of John's but I recently remember hearing that the Bible mentions that Mary was related to John's mother. I wanted to ask what historians think of this. Also, why isn't this mentioned among Christians? I've never heard it until recently. Also, are James and Jesus literal brothers as the Gospels imply?
Jesus' mother, Mary, and John's mother, Elizabeth, were relatives (Luke 1:36). The old King James Version of the Bible says they were cousins, but the word "cousin" used to mean any relative in the 17th century when the KJV was written.
The evangelist Luke wrote of a biological relationship between the mothers of Jesus and John the Baptist. Reportedly, the angel Gabriel told Mary “your relative Elizabeth in her old age has also conceived a son.” (Lk 1:36). The original Greek text does not tell us exactly how they were related, but common tradition has held that because their mothers were related, Jesus and John must have been cousins. After this detail, however, nothing else in our Bible mentions any interactions between the two during their childhoods. Indeed, the description of Jesus’ baptism by John the Baptist suggests that might have been their first meeting, which could then explain John’s question about Jesus’ true identity (see Luke 7:18-23). One possible explanation for this seeming lack of relationship during their boyhood years is Luke’s detail that John “was in the wilderness until the day he appeared publicly to Israel.” (Lk 1:80). The apocryphal “Infancy Gospel of James,” written perhaps in the mid-second century, mentions that John’s mother Elizabeth fled with him to the hill country to avoid Herod’s slaughter of the innocent boys born around the time of Jesus.
Edit: Added second source
/u/ZoomasterEddie already answered about the connection between Mary and Elizabeth in the Bible, and is spot on.
As for James and Jesus, the main point of contention is the term 'brother' (ἀδελφός) which very naturally can mean 'brother', but just as our word it is capable of extension to other types of relations.
James, at least the one I believe you are referring to, is called the brother of the Lord, as in Galatians 1:19. It's clear from the NT texts that there are quite a few 'James' running around (actually, in Greek they are all Jacob), so generally they are distinguished by terms that make them identifiable.
The R.Catholic church holds the belief that Mary remained a virgin after the birth of Jesus and so bore no other children; on their reading 'brother' when talking about a brother of Jesus is to be understood as 'cousin'. Most non-R.Catholic scholars, religious or otherwise, find this to be a dubious way to read those references.
In which case the James in question would be a brother (if Joseph is the father of Jesus) or a half-brother of Jesus (if Joseph wasn't).
As for your question "why isn't this mentioned among Christians?" I would only say, "which Christians, where?" because these are pretty widely known details.