Today's Gospel reading (Mt 13:54-58), I guess answers the question.
But every Catholic and Orthodox Christian knows that this is wrong. In fact Catholic Christians and Orthodox Christians believe that Jesus didn't have any biological brothers and sisters but maybe half-brothers and half-sisters. We believe that Mary, always remained a virgin and so we call her Ever Virgin.
Many Protestant Christians refer to the verse above and similar ones to justify that Jesus had siblings. But within these passages from the Bible lies a hint that Protestant interpretation is wrong.
If we interpret these verse literally, then we are also at risk of admitting that Jesus was a mere mortal. What's that? The first sentence of verse 55 is, "This is the carpenter's son, surely?"
But all Christians know that Joseph was only Jesus's foster father and not his biological father. So how should we interpret the remaining verses that actually name Jesus' brothers and sisters?
You shouldn't see these verses with the lens of 21st Century Western culture. Instead you should look at the language of ancient times. In Hebrew, Indian and even European cultures, it was common to refer to a cousin as brother or sister. Even till today, the term used for cousin in South Asia, is either brother or sister.
There protoevangelium of James (which never made it into the Bible) says that Joseph was an old man who had kids from a previous marriage. These could be the brothers and sisters that several verses in the Bible refer to.
Now, let's assume that Jesus really did have biological siblings, then in ancient cultures and even today in most underdeveloped economies, a widow is cared for by the surviving siblings. So, it would have been a major insult for Jesus to ask John to take care of Mary after His death. Anyway, why should Jesus ask John to look after Mary? It's a totally stupid request from a non-spiritual view. He could have asked Mary Magdelene who was also at Golgotha.
John was like a brother to Jesus. That's why the Gospel of John frequently has John referring to himself as the disciple Jesus loved. And because John was Jesus' spiritual brother, Jesus asked him to look after her and instantly Jesus made her our mother too, just as she became the spiritual mother of John and all the Apostles and disciples. This is the reason why Catholics and Orthodox Christians call Mary, Mother Mary.
Every verse in the Bible has a message for our soul. Nothing there is mundane. It would be out of place for the Holy Spirit to inspire the authors of Scripture to record silly details that are merely for knowledge. Our salvation is not furthered one bit by knowing that Jesus had biological brothers and sisters. For the advancement of knowledge the Holy Spirit will use historians and scientists.
The purpose of these verses is to merely indicate that people couldn't accept Jesus because they thought they knew him as a mere carpenter who had human relatives. They wanted a Messiah who was wholly spiritual like YHWH.