The Bible refers to Jesus as having “brothers”—e.g., “He was still addressing the crowds when His mother and His brothers appeared outside to speak with Him.”
Saint Jerome (the translator of the Bible into the latin vulgate) was a champion of the defense of Mary’s prepetual virginity in the fifth century. The Roman Catholic church subscribes to this belief stating that Jesus’ brothers are not Mary’s children. They might have been Joseph’s from a previous marriage, adopted, or actually cousins who lived in the same household.
Many protestant churches teach that Jesus’ brothers are Mary’s other male children.
What is the Orthodox Church’s thinking concerning the prepetual virginity of Mary?
Concerning Mary’s perpetual virginity, the Orthodox Church also holds firmly to this doctrine. We firmly believe that Mary remained a virgin after she gave birth to Christ, while we reject the notion that she had other children after Him.
In the case of the word “brother,” it is clear that it has meanings other than biological, as you note.