Nothing in Hebrew 7 indicates that Blessed Virgin Mary is not Christ's mother or that we do not know Christ through her only. He did not have two mothers.
If you are curious why Leo XIII said that, perhaps you should read what he said because I am sure he articulated it better than I can. In fact, I will appreciate a link or a full quote with context if you have it as well.
Here is what God says that Mary said about her role. When did God ever say differently?
Luke 1:38 And Mary said, Behold the handmaid( bond servant, slave) of the Lord; be it unto me according to thy word. And the angel departed from her.
We know Christ only through her? How does that work?
Please show me then that he is not the Great High Priest, that his mother is! That her, by being His Mother, overshadows His work on the cross!
Jesus saith unto him, I am the way , the truth, and the life: no man cometh unto the Father, but by me.
Where in scripture does Jesus say "No man cometh unto me but by my mother"?
John 14:13 And whatsoever ye shall ask in my name, that will I do, that the Father may be glorified in the Son.
14:14 If ye shall ask any thing in my name, I will do it.
Where is the mention of his mother?