The Episode 19 of "Castle" Season 6 is titled, "The Greater Good." Captain Gates' sister features in this episode. The episode seems to be inspired by Leonardo DiCaprio's movie, "The Wolf of Wall Street," as the preview of the episode says, "a wall street wolf's murder exposes a case of greed gone wild."
The murdered victim is not just another Wall Street wolf, but was apparently working for the U.S. Attorney's office. In the sneak peek, Lanie can be seen telling Castle and Beckett that the victim was wearing a wire. The killer seems to have removed the wire from his body after shooting him.
The routine murder investigation will become complicated, with the arrival of two women from U.S Attorney's office. The sneak peek shows Captain Gates revealing to Castle and Beckett that one of the women is her sister, Elizabeth Weston. Salli Richardson-Whitfield has been roped-in to play the estranged sister of Captain Gates. This murder investigation will for sure reveal interesting and exciting twists and turns, before Castle and Beckett catch the killer and find the motive.
In the preview episode, Castle had promised Beckett that their married life will never be boring. One can expect some romantic moments between these two, as "Castle" Season 6 is heading towards its finale.
According to the synopsis of "The Greater Good," the murdered man was working for the U.S. Attorney's office as an undercover informant. Two estranged sisters under the same roof is set to make things worse, before mending their relationship to some extent. It will be interesting to see how Castle and Beckett solve the case, without letting Weston interfere too much.
Official Synopsis of "Castle" Season 6 Episode 19, "The Greater Good": "Investigating the death of a Wall Street trader leads Castle and Beckett to discover that the victim was an undercover informant for the U.S. Attorney's office. To make matters worse, the U.S. attorney involved is Captain Gates' estranged sister."