They are both half sisters. Nick (left) is the daughter of a police officer and of an intellectual family. She is now a demon slaying avatar and it's questionable if what she sees is reality or it is a result of internal turmoil and demons. She wields the Sword of Ruth that is the Truth-Seeker. Shin (right) is half-Japanese and from the same mother. She works now as an Interpol agent (but in the deepest crevasses of secrecy that Nick used to be in).
It's not known if the sisters do care about each other or are trying to kill each other. There's an uneasy truce about them. Both are proficient in hand-to-hand combat and gunplay. Nick is more proficient in swordplay but she's not too bad with other weapons. Shin prefers a small daggerlet.
Still working out the kinks in the story, but I'm quite happy with the development so far.
*They are going through a major update.