Yakuza Goddess of Mercy: Mistress Jingi

Seigen is an orphan monk. His mother was dead when he saw the day thanks to a monk, and then was raised at a monastery. One day, he helps a woman to escape some threatening yakuzas. She seems to be very thankfull, so she greet him with her body. Later Seigen learns that she is actually his halfsister, they have the same (yakuza) father.