Jesus had at least four brothers and two sisters. The names of the brothers are James, Joses (or Joseph), Jude (or Juda, Judas), and Simon). They can be found in the Gospels of Matthew and Mark.
"Is not this the carpenter's son? is not his mother called Mary? and his brethren, James, and Joses, and Simon, and Judas? And his sisters, are they not all with us? Whence then hath this man all these things?" (Matthew 13:55)
"Is not this the carpenter, the son of Mary, the brother of James, and Joses, and of Juda, and Simon? and are not his siters here with us? And they were offended at him." (Mark 6:3)
Other less specific references to Jesus' siblings can be found in Matthew 12:46, Mark 3:31, and John 11:3.
Some apologetics argue either that Jesus had no siblings or that any brothers and sisters he may have had were half brother/sisters that were from Joseph's previous wife before he married Mary. However, both the Gospels of Matthew and Luke clarify that Jesus was Mary's "first born son" and thus was the oldest of his siblings.
"And knew her not till she had brought forth her firstborn son: and he called his name Jesus." (Matthew 1:25).
"And he came by the Spirit into the temple: and when the parents brought in the child Jesus, to do for him after the custom of the law," (Luke 2:27)
Note: By law, any first-born must be devoted to Yahveh: You shall set apart to the Lord all that open the womb (Exodus 13:12), which is the text quoted by Luke.
The Catholic Church argues that the Greek word adelphos used to describe Jesus brothers/sisters can either mean "brother" or "relative". The word contains some of the same concepts as brother in its range of meaning. The Catholic view is that the verses refer to Jesus' "cousins". The idea of that Jesus had cousins, not siblings, was popularized by Jerome at the turn of the 5th century.
The problem is that the Greek word anepsios specifically means "cousin". So one would have to ask... "Why wasn't anepsios used if that is what was meant?" Doesn't make sense.