The System (Fire Universe #4)
Nicole Pyland

Carina Whitlock is the top ADA in her office and is on her way to becoming the next district attorney when she is handed the case of Marin May, Kieran’s twin sister. It seems like a fairly open-and-shut case until she spends time with Kieran and finds herself not only believing that Marin might not have committed the crime, but also that Kieran might be more than someone she’s supposed to know through a case.
All the evidence points to Marin being the killer and maybe much more than just that, but Kieran can’t stop trying to help her sister and becoming friends and maybe more with the ADA assigned to prosecute the case.