Tango & Cash

Beverly Hills LAPD Lieutenant Raymond Tango (Sylvester Stallone) and Downtown Los Angeles Lieutenant Gabriel Cash (Kurt Russell) have earned themselves reputations for disrupting crime lord Yves Perret's (Jack Palance) smuggling operation in their respective jurisdictions. One day, they are informed of a drug deal taking place later that night. Both detectives meet for the first time at the location, but discover a dead body that is wire-tapped before the FBI arrive and surround the duo. Agent Wyler (Lewis Arquette) finds Cash's backup Walther PPK pistol on the floor with a silencer attached and arrests both Cash and Tango. At their murder trial, Tango and Cash are incriminated by an audio tape, secretly given to Wyler by Perret's henchman Requin and verified in court by an audio expert, which appears to reveal them shooting the undercover FBI agent after discussing a drug purchase. They plead no contest to a lesser charge in exchange for reduced sentences in a minimum-security prison, but are instead transported to a maximum-security prison to be housed with many of the criminals they arrested in the past.
Once in prison, Tango and Cash are rousted from their bunks and tortured by Requin and a gang of prisoners until Matt Sokowski, the assistant warden and Cash's former commanding officer, rescues them. Sokowski recommends that they escape and provides them with a plan, but Tango refuses to go along with it. When Cash tries to escape, he finds Sokowski murdered and is attacked by prisoners. Tango rescues him and the duo escape. Once outside the prison walls, they proceed to go their separate ways when Tango tells Cash that should he need to contact him, he is to go to the Cleopatra Club and look for "Katherine."
The detectives then visit the witnesses who framed them in court. Wyler admits to Tango that Requin was in charge of the setup, and Cash discovers that Skinner, the audio expert, made the incriminating tape himself. Cash finds Katherine, who helps him escape the night club as police move in on him on female disguise. Later that night, Tango reunites with Cash, who discovers that Katherine is Tango's younger sister. The duo are met at Katherine's house by Tango's commanding officer, Schroeder, who gives them Requin's address and tells them they have 24 hours to find out who Requin works for. Tango and Cash apprehend Requin and trick him into telling them Perret's name. Armed with a high-tech assault vehicle loaned to them by Cash's weapons expert friend Owen, the duo storm into Perret's headquarters to confront the crime lord. At this point, Perret, who has kidnapped Katherine, starts a timer that will trigger the building's automatic self-destruct procedure. After killing Perret's core security personnel, Tango and Cash are confronted by Requin, who is holding Katherine at knifepoint but throws her aside to fight the detectives hand-to-hand with the help of another henchman. The detectives defeat the two henchmen and when Perret appears, holding a gun to Katherine's head, they kill him and leave with Katherine just before the building explodes. Afterward, they joke half-seriously about Cash's desire to date Katherine before they have themselves vindicated the next day.

Question: What is Katherine's relationship to Tango?
Answer: Younger sister