They are average Homicide detectives that from time to time reopen old cases, and they often talk about recent cases they closed before they went cold and are rarely shown through the series.
When one of them is about to run from a murder scene, they call each other by their first names before the friend runs away.
She and her sister were foster kids, and their foster mother was much more attached to the sister if understandably so — she initially tried to connect with both of them, but the victim, jaded by past experience, pushed her away.