Not much of a mystery.

The prologue starts with portret of Christa. She's painted as a not very likeable person, so it's not really a surprise she is killed.

The next chapter shows Heather at work. She's painted as a much nicer person than Christa. In walks a detective who wants to ask a few questions regarding Christa's death. Heather is sort of a suspect and decides to investigated. Conveniently helped by Christa's brother

Spoiler

who gives her a key to the shop, enabling Heather to look around.

Why the brother would do the things he does is not explained. Did he have a past relationship with Heather? Any reason why he needed her to know he did not kill his sister? As far as the story told me Heather is the former boss of Billy's sister. His actions are convenient for Heather, but make no sense in the story.

Same with Christa's mom airing the family's dirty laundry over the phone with Heather. Same with Detective Sheppard sharing case details with Heather.

The plot was interesting though, but it could have been much better executed.