First it is important to note that the Bible does not say that these "brothers and sisters" of Jesus were children of Mary.Second, the word for brother (or sister), adelphos (adelpha) in Greek, denotes a brother or sister, or near kinsman. Aramaic and other semitic languages could not distinguish between a blood brother or sister and a cousin, for example. Hence, John the Baptist, a cousin of Jesus (the son of Elizabeth, cousin of Mary) would be called "a brother (adelphos) of Jesus." In the plural, the word means a community based on identity of origin or life. Additionally, the word adelphos is used for (1) male children of the same parents (Mt 1:2); (2) male descendants of the same parents (Acts 7:23); (3) male children of the same mother (Gal 1:19); (4) people of the same nationality (Acts 3:17); (5) any man, a neighbor (Lk 10:29); (6) persons united by a common interest (Mt 5:47); (7) persons united by a common calling (Rev 22:9); (8) mankind (Mt 25:40); (9) the disciples (Mt 23:8); and (10) believers (Mt 23:8). (From Vine's Expository Dictionary of Biblical Words, Thomas Nelson, Publisher.)
Protestant reformers like Martin Luther and John Calvin also believed in the perpetual virginity of Mary.
God bless.