I think you are looking for Yakuza Woman by QuietSerenade on fanfiction.net. I hope you have found it by now, but if not I hope this is the story.

There was a story, I'm not sure where I read it, but the story was bassed in the future when the world had gone into a major deptession and demons were revealed and they saved society by taking over in the form of gangs.
Nobody knows Kagome is a girl except her family. Her parents made sure that everyone around her thought she was a boy and enroled her in a boys accadame.
But something happened to all the priests and priestess and girls were a rariety. Kagome had three brothers and she had to make a deal with Sesshomarus gang to be able to save the shrine. So she goes to work for him as his own personal monk/priest.
I think it was called Yakuza Girl, but when i search for it nothing comes up.