Zeoticus and Venelana had a pair of twins miraculously, however, they were born human and immune to Evil Pieces. So they had to hide them, Zeoticus dropped them off at a Orphanage, planning to pick them back up in 10 years if they hadn't been adopted.

However the Orphanage was already full so the two were split up and went to different Orphanages, Maria was taken to the Vatican because of her Subspecies Blade Blacksmith Sacred gear, one priest discovered her Power Of Destruction, and told her to never use it, to protect her.

The Boy, Markus, went to a crummy Boys home, where he was picked on, however, he ran away after looking through the records, and saw that he had a Twin sister out there somewhere. His Sacred Gear is a Subspecies Sword Birth. 

After Years of Tracking, learning, Training, and encountering the Supernatural, Markus finally Tracked down his sister in Russia dealing with a Stray Devil that had reached an uncontrolled level, one that would easily kill Maria. Reason why is that someone in the church, someone who figured out that Maria Has the Power Of Destruction, wanted her dead and tampered with information on the stray.

The Subspecies sacred gear they have, they can make more then just swords, such as armor, shields, lances, and the like. And their power reaches that of true Holy and Demonic swords.