Brethren of the Lord means in the bible: The title was given to James and Joses. 13:55. See also Matt. 12:46. Mark 6:3 and Luke 8:19. John 2:12; John 2:32; Acts 1:14 and 1 Cor. 9:5; Gal. 1:19. These were the children of Joseph und Mary who were born shortly after Jesus’s birth. Matt. 13:56 reveals that there were sisters. Although the number of sisters isn’t specified in Greek, it appears that more than two were present. The exact number of girls is unknown so the KJV presents it as “all his siblings”.
(in Bible Dictionary)