It is believed by most scholars that Joseph, Jesus’ step dad, was quite a bit older than Mary, and had been married to another woman who had passed away. They believe that Joseph had children from that previous marriage, and that he brought them into his marriage with Mary.

That this could be so is isn’t found in Scripture, but the Bible clearly states that Jesus had brothers and sisters. However,  they couldn’t have been related by blood to Jesus; for if Mary had been their birth mother, we would have quite a different story than what is recorded.

For one thing, it appears that Joseph had died by the time Jesus began His public ministry. This is assumed because there isn’t any mention of Joseph being alive at that time and afterwards. If all the children had been Mary’s offspring, then Jesus would have been the eldest brother. That meant, according to custom and duty, that Jesus would have taken over the family business upon Joseph’s death. He wouldn’t have left home to go away preaching.

The fact that He did, tells us that there were older brothers around to run the carpentry business. Because Mary was a virgin when she gave birth to Jesus, that means the older brothers had to have been step-brothers.

Another thing that comes to mind is the scene in which Jesus is hanging on the cross. To the Apostle John standing at the foot of the cross, Jesus obligated him to take care of His mother. This too suggests that Mary was not the birth mother of the other children. For if she had been, then automatically she would have fallen into their care.

Beyond Scripture, we cannot be certain if Mary did or didn’t birth any other children. However, based within the framework of the Scripture story; most of the scholars I’ve heard or read, conclude that Jesus was Mary’s only biological child.