Why do many people believe Jesus had siblings, in light of His instruction at the time of the crucifixion to the disciple John to take care of his mother Mary (John 19:27)?

Many people believe Jesus had siblings in light of multiple verses of scripture which speak of them; and they believe it notwithstanding the light of John 19:27.

The light of John 19:27 alleged by the question, however, is darkness.

Jesus never instructed his disciple John, or any other disciple, to take care of his mother Mary.

Jesus said to the disciple,” “Behold your mother!”

The evangelist finished verse 27 not by saying, “And from that hour that disciple took her unto his own (home),” but by saying, “And from that hour that disciple took her for his own (mother).”

This is the true light of John 19:27, and I allege it only in my own name.

The words of Jesus revealed the creation of new relationships. Explicitly, a mother gained a son,and a son gained a mother. Implicitly, but more to the point, Jesus gained his brother.

The disciple whom Jesus loved, you see, was the one that had lifted up his heel against him, the one that was lost, whom Jesus came to save.