"While Jesus was still talking to the crowd, his mother and brothers stood outside, wanting to speak to him" Matthew 12:46
"Isn't this the carpenter's son? Isn't his mother's name Mary, and aren't his brothers James, Joseph, Simon and Judas?" Matthew 13:55
"Now Jesus' mother and brothers came to see him, but they were not able to get near him because of the crowd." Luke 8:19.
"After this he went down to Capernaum with his mother and brothers and his disciples. There they stayed for a few days"
"They all joined together constantly in prayer, along with the women and Mary the mother of Jesus, and with his brothers" Acts 1:14.
These are some of the scriptural references indicating that Jesus had siblings. However, there is no reference to a twin. On the contrary-
Matthew 1:23 tells us that Mary gave birth to a son (singular not plural)-
""The virgin will be with child and will give birth to a son, and they will call him Immanuel" —which means, 'God with us' "
All other siblings that Jesus may have had came after Mary and Joseph were wed and after the birth of Jesus. Biologically- they would be considered half brothers.