FrecklesdaBunny
The main character, Lilith, had an abusive childhood, so she took her sister and left at the age of 14. She had to find some means of supporting both herself and her sister, so she chose to do something she would never have dreamt of doing as a younger child herself. (I can't really write much more without spilling all the beans, so you'll just have to read and find out the rest~)
WARNING: I've included some slightly graphic violence, but it's not very descriptive, so I'm assuming it doesn't exactly count as 'mature', but nevertheless, read at your own risk~ (If you're going by the cover, believe me, it isn't gun violence on every page, and it only really appears a couple of times)