Corrected entry: **MAJOR SPOILERS** It makes no sense that Detective Bailey would be allowed to work on a case involving Samantha. His superiors would quickly see that she was involved in the death of his son Richie and would never allow him to work on the case.
Correction: Bailey and his children are all serial killing psychopaths that have lied about being related to each other, lied about killing anyone, lied about starting horrible rumors about Samantha online, and lied about Kirby being the killer. Nothing Bailey says to the main cast before the reveal should be taken at face value by the audience, and it's clear the family comes from some other high amount of wealth outside of the NYPD. Also by definition the film itself provides and explanation for why he is on the case (whether it be credible or not is irrelevant) and that alone by definition does not make this a plot hole.
Correction: It seems evident that Richie had assumed a whole new identity in order to seduce Sam and start his killing spree, and that Detective Bailey subsequently used his connections in law enforcement to further ensure their familial link wasn't discovered. Bailey also mentions that the previous detective assigned to the case offered it to him specifically because it involved his daughter. This all may strain credulity but it isn't a plot hole that's simply overlooked or ignored.
TonyPH