The prayers are supposed to be for Mary to pray for us to be Blessed by God the Father, not to bless us herself. And she (or her image) is not to be worshipped (idolatry), only honored.
On the other hand, though she was virgin at Jesus' birth, he had siblings, and in Jewish tradition, IIRC, an unconsummated marriage is no marriage at all. That has never been adequately explained, to me anyway.
Source??
Well, there are no admonitions in the Word to pray to Mary for anything; nor are there any indications she can do anything for us as catholics believe.
Catholics can claim they don't worship Mary, but their actions show otherwise as evidenced by this thread, and their prayers to Mary, the shrines to Mary, the writings about Mary, etc.