The Legend of Cagney, BOOK I

Cagney is the master of disguise, no one knows what she looks like except for her little sister Tamika. Cagney is an assassin that lives a double life, she is also a NYPD POLICE OFFICER. There are four women that all hang out together, all four are NYPD OFFICERS. Which one of these women is really Cagney? If you like murder and mystery you are going to love The Legend of Cagney