It always amazes me when I hear or meet someone that is so skeptical of Christianity, that they even doubt the existence of Jesus. I can understand when people have a hard time believing that Jesus is God and that He rise from the dead, but it is baffling to me when someone is so skeptical that they do not even believe Jesus was a historical person. No matter how baffled I am by their skepticism, I am reminded that I need to have evidence to make the case that Jesus was an actual person in history. One of the pieces of evidence that makes a good case for Jesus being a historical person is the independent ancient sources that mention Jesus' family in their writings.
 
THE NEW TESTAMENT SOURCES
The first group of independent sources are the manuscripts that we now call the New Testament. Throughout the Gospels, Acts, and the Epistles, Jesus is the figure that everything revolves around. These writings mention things that Jesus did and said, who His family was, and most notably how He died and was resurrected. The best sources we have within the New Testament are the letters of Paul. In his book Jesus Interrupted, the agnostic New Testament scholar Bart Ehrman says this:
“The seven letters that virtually all scholars agree Paul wrote—the so-called undisputed Pauline epistles—are Romans, 1 and 2 Corinthians, Galatians, Philippians, 1 Thessalonians, and Philemon.”[1]
Since most scholars agree that Paul wrote these letters, let’s investigate them to see if we can find strong evidence to the existence of a real, historical Jesus.
In Galatians 1:18-19, Paul says:
Then three years later I went up to Jerusalem to become acquainted with Cephas, and stayed
with him fifteen days. But I did not see any other of the apostles except James, the Lord’s
brother.”
The fact that Paul mentions James the Lord’s brother, is great evidence for the existence of Jesus. The James being spoken of here is James, the son of Mary and Joseph, not James the disciple of Jesus who was the brother of John. Because Paul mentions that he met the brother of Jesus, doesn’t it follow that Jesus probably existed? The meeting that Paul is referring to in these verses occurred somewhere between 35-36 AD. This means that this man James was believed to be Jesus’ brother very early in Christian history. The manner in which Paul mentions meeting James is important as well. Paul does not mention this as an argument to prove the existence of Jesus, but he presents it as a secondary, non-important detail of his trip. This adds validity to the truth of his statement, because he is not adamantly trying to argue or prove anything by this statement. It is merely a detail of his time in Jerusalem. This is also an interesting side note for Paul to mention when we take Mark 6:3 into account.
“Is not this the carpenter, the son of Mary, and brother of James and Joses and Judas and Simon? Are not His sisters here with us?” And they took offense at Him. –Mark 6:3
In this passage, Mark mentions Jesus’ siblings and specifically names James as being one of them. This statement does not contradict Paul’s statement about James, but reinforces it. It seems that the names of Jesus brothers were known among the writers of the New Testament.

Another statement about Jesus’ brothers is mentioned in 1 Corinthians, one of the so-called undisputed letters of Paul according to Bart Ehrman.
“Do we not have a right to take along a believing wife, even as the rest of the apostles and the brothers of the Lord and Cephas?” -1 Corinthians 9:5
The mention again of the brothers of the Lord is puzzling if a historical Jesus did not actually exist. The fact that Paul mentions meeting James and knowing that Jesus brothers had wives is good evidence for believing that Jesus existed.
 
NON-CHRISTIAN SOURCES
In addition to the New Testament documents, we also have a 2nd century Non-Christian source, Josephus, that mentions James, the brother of Jesus.
Josephus says:

Festus was now dead, and Albinus was but upon the road; so he assembled the sanhedrin of judges, and brought before them the brother of Jesus, who was called Christ, whose name was James, and some others, [or, some of his companions]; and when he had formed an accusation against them as breakers of the law, he delivered them to be stoned; -Antiquities of the Jews 20.200

The fact that Josephus was not a Christian adds tremendous weight to his statement about James, the brother of Jesus who was called the Christ. Why would he write this down if he did not believe it to be history? Also, I generally think it is a good rule of thumb to take more seriously the statements of multiple sources close to the times of the event, rather than people two-thousand years removed. 

Paul, Mark and Josephus all mention Jesus’ brothers, particularly James, which gives us great historical evidence that Jesus must have existed. If he did not exist, where would these people claiming to be his family have come from? Also, how could a group of people make up a conspiracy about a man who never existed and convince others that they were his family? There are many good reasons to believe that Jesus was a historical person, but the fact that He had brothers, who Paul had met, makes His existence pretty obvious.
 
HOW TO SHARE THIS
So what should we do with this information? Here are a few ways you can share these arguments with your skeptical friends.

When someone claims that Jesus didn’t exist, do not get defensive. You did not make the claim, so you do not have to defend your position. They need to defend their position. Ask them what evidence they have for their belief that Jesus did not exist. Keep asking questions to see how they have reasoned out, or haven’t reasoned out, their position. After asking them questions for some time, I would offer them the evidence about Jesus’ brothers from this blog. After you have presented the evidence continue to ask them questions. How would a man who did not exist have brothers that were well known? How could a conspiracy arise about a non-existent man and that included people lying about being his family in the town where he was supposedly killed as early as 5-6 years after his death? Help your friends think through these questions and others like them. Once a person believes that Jesus existed as a historical person, you can then start the discussion about who this man claimed to be and His resurrection from the dead.

Remember that we are called to give good arguments and evidences for what we believe as we help others see that their speculations do not hold up to the truth.
“We are destroying speculations and every lofty thing raised up against the knowledge of God, and we are taking every thought captive to the obedience of Christ” -2 Cor. 10:5
[1] Ehrman, Bart. Jesus Interrupted. Pg. 63