She was to give birth to the 'WORD', Jesus Christ. Blessed art thou among women'. John the Baptist had not yet appeared to start the Sacrament of Baptism. She had to be 'Immaculately Conceived' to satisfy the fact that GOD cannot coexist with sin. At first it looks like Jesus had brothers and sisters.
Lets look at the definitions.
They were 'fellow men'. This makes the word 'brother' a non-argument. The verse means, 'the gulf between creatures and GOD is too wide to be bridged unless GOD Himself transforms us'. B. Jesus Christ ascended into Heaven body and soul. This verse could apply to Mary as well as to the 'Messiah'.