The book of Mark

My understanding is that "Mark" is the same John-Mark mentioned in the book of Acts. He was an acquaintance or companion of the apostle Peter and probably of the other original apostles, possibly also a mentee of Peter's. Peter refers to Mark (or "Marcus") as his "son" in 1st Peter 5:13, just as Paul referred to Timothy (see the beginning of 1 & 2nd Timothy), which suggests a special relationship between Peter and Mark, like the relationship Paul had with Timothy.

The gospel of Mark was probably written from Peter's perspective, as dictated to John-Mark. Peter was not a learned man (Acts 4:13) and was probably illiterate and had other people write his letters for him. (1 Peter was written by a brother named Sylvanus at Peter's dictation. See 1 Peter 5:12)

(Luke was not an apostle either but a companion of Paul's, a doctor by occupation. He wrote the gospel of Luke, the book of Acts, and possibly Hebrews at Paul's dictation--because of Paul's poor eyesight due to the demon ("thorn in the flesh") God had given Paul to keep him humble.)

Mark's gospel was probably written from Peter's perspective; Matthew was from Matthew's, John from John's, and Luke's was a collection of multiple eyewitness accounts, written for a person of importance by the name of Theophilus (see Luke 1:1-4 & Acts 1:1).

Mark may have been among the disciples during Jesus' ministry, following along with them, though not necessarily a disciple himself. (He may have been that young man who fled away naked at Jesus' arrest, mentioned in Mark 14:51,52.)

Well that's interesting, Sister. You've obviously done a lot of research into this subject. I looked back into the Scriptures and I noticed a few things that I thought I would share:

In the gospel of Matthew (who is also called Levi), the groups of brothers are noted as such. For example (in Matthew 10):

...Simon, who is called Peter, and Andrew his brother; James the son of Zebedee, and John his brother.

Then the other apostles are mentioned, without any mention of family ties between Matthew, James and Thaddaeus/Lebbaeus, which it seems likely that Matthew (or one of the other authors of the gospels) would have noted if there was one, as he did with Peter and Andrew and James and John.

In the gospel of Luke, Thaddaeus/Lebbaeus is referred to as Judas, but he is not referred to as the son of Alphaeus but rather as the son of a James (Luke 6:16 ESV). (The KJV translates Luke 6:16 differently than other translations; it translates it as "brother of" while most others translate it as "son of"). 

Only the apostle James is consistently described as "the son of Alphaeus", as a specific title of identification, in any of the gospels, in the parallel Scriptures about the choosing of the twelve disciples. In every gospel, when the Lord's choosing of His 12 disciples is described, James the son of Alphaeus is exlusively identified by that title. If the same Alphaeus was also the father of Matthew/Levi, it seems likely to me that Matthew and James' brotherhood would have been mentioned somewhere, as that of Peter and Andrew and James and John is. Levi's father's name is only mentioned once, in Mark 2:14, but later in the same epistle, where Levi is called Matthew and is mentioned among the twelve apostles, his father's name is not mentioned, but James' is ("the son of Alphaeus", as always), probably to show that the two of them were not related, which might have been assumed if Matthew had also been described as "the son of Alphaeus".

(A lot of people had the same name back then, so the fact that James and Levi/Matthew both had fathers named Alphaeus doesn't necessarily mean that they were brothers. They could have just had fathers with the same name.)

As for James and Jude being brothers, the Jude who wrote the epistle of Jude was not Judas the son of James (Luke 6:16) or a brother of James the son of Alphaeus, but the brother of Jesus. Jesus had natural brothers and sisters, and two of His brothers were named James and Judas.

"Is not this the carpenter's son? Is not His mother called Mary? And are not His brothers James and Joseph and Simon and Judas? And are not all His sisters with us?" (Matthew 13:55,56)

Jesus' brothers weren't initially believers (John 7:2-5), nor apostles, but His brothers James and Judas (aka Jude) eventually became believers---perhaps after Jesus' resurrection---and James later became an apostle of importance in the church in Jerusalem. He is mentioned in Galatians chapters 1 (verse 19) and 2, and is most likely the James of Acts 15:13, given Paul's description of him as a pillar of the church in Jerusalem in Paul's epistle to the Galatians.

For the same reason, he is probably also the author of the epistle of James, rather than James the son of Alphaeus or James the brother of John, who was killed with the sword early on in the apostles' ministry (Acts 12:1,2). Jude, the author of the epistle of Jude, is the brother of this James, the brother of Jesus, not James son of Alphaeus.

I am going to leave this for a bit and check out some of the things that sister lights pointed out.  It's going to take a while, and it gives me a head-ache to tell you the truth! 

And, yes I am under the impression that Mark is the youngest apostle John,.... in my spirit I know he is, but I need more conclusive evidence to make this an absolute fact, and if the Lord grant me this, I will share, and if he shows me he is definitely not, I will share also because I don't want to put any errors out there.  When this was all revealed to me about a year ago, I was so excited, and it was just one thing after another that was unfolding. I still have a big list of things to share, but will do that after double checking again.