Board Thread:Questions and Answers/@comment-25669979-20161220025242/@comment-25942629-20170107165148

Shinoa and Mahiru don't necessarily have the same mother. Shikama Doji was passed to Shinoa. It was implied that Asuramaru was passed to Mahiru from her mother.

Later on, Mahiru takes Shikama Doji from Shinoa and begins to go mad because there are two demons attacking her sanity at once. Although Mahiru takes Shikama Doji for awhile, she was not born with her.