Star Wars Episode VI: Return of the Jedi

Luke Skywalker initiates a plan to rescue Han Solo from the crime lord Jabba the Hutt with the help of Princess Leia, Lando Calrissian, Chewbacca, C-3PO, and R2-D2. Leia infiltrates Jabba's palace on Tatooine, disguised as a bounty hunter with Chewbacca as her prisoner. Lando is already there, disguised as a guard. Leia releases Han from the carbonite, but she is captured and enslaved. Luke arrives soon afterward but, after a tense standoff with Jabba and a battle with his rancor, is captured. Jabba sentences him, Han and Chewbacca to death, planning to feed them to the Sarlacc, a pit monster. They are taken to the Great Pit of Carkoon, the Sarlacc's nesting ground. Luke, with R2-D2's help, frees himself and battles Jabba's guards. During the chaos, Boba Fett attempts to attack Luke, but Han, temporarily blinded from the carbonite, inadvertently knocks him into the Sarlacc pit. Meanwhile, Leia strangles Jabba to death, and Luke destroys Jabba's sail barge as the group escapes. While the others rendezvous with the Rebel Alliance, Luke returns to Dagobah, where he finds that Yoda is on his deathbed. Before he dies, Yoda confirms that Darth Vader, once known as Anakin Skywalker, is Luke's father, and that "there is another". The ghost of Obi-Wan Kenobi confirms that this "other" is Luke's twin sister: Leia. Obi-Wan tells Luke that he must fight Vader again to defeat the Empire. Obi-Wan also warns Luke to keep his emotions in check, as his anger could lead him to the Dark Side.
The Rebels learn that the Empire has been constructing a second Death Star under the direct supervision of the Emperor himself. As the station is protected by an energy shield, Han leads a strike team to destroy the shield generator on the forest moon of Endor; doing so would allow a squadron of starfighters to destroy the Death Star. The strike team, accompanied by Luke and Leia, travels to Endor in a stolen Imperial shuttle. On Endor, Luke and his companions encounter a tribe of Ewoks and, after an initial conflict, gain their trust. Later, Luke tells Leia that she is his sister, Vader is their father, and he must go and confront him. Luke surrenders to Imperial troops and is taken to Vader. Luke tries to convince Vader to turn from the dark side of the Force, but fails.
Vader takes Luke to the new Death Star to meet the Emperor, who is intent on turning him to the dark side. The Emperor reveals that the Death Star is fully operational and the Rebel fleet will fall into a trap. On Endor, Han's strike team is captured by Imperial forces, but a surprise counterattack by the Ewoks allows the Rebels to battle the Imperials. Meanwhile, Lando, piloting the Millennium Falcon, leads the Rebel fleet to the Death Star, only to find that the station's shield is still active and the Imperial fleet is waiting for them. The Emperor tempts Luke to give in to his anger and join him. Luke engages Vader in a lightsaber duel. Vader senses that Luke has a sister and suggests turning her to the dark side. Enraged, Luke overpowers Vader and severs his father's prosthetic right hand. Upon seeing the remnants of Vader's prosthetic, he sees a parallel between himself and Vader, and fears he will become like Vader. The Emperor tells Luke to kill Vader and take his place, but Luke refuses, declaring himself a Jedi as his father had been. On Endor, the strike team defeats the Imperial forces and destroys the shield generator, allowing the Rebel fleet to launch their assault on the Death Star. Simultaneously, the Emperor tortures Luke with Force lightning. Unwilling to let his son die, Vader throws the Emperor down the Death Star reactor shaft, killing him, but is mortally wounded in the process. He asks Luke to remove his mask, and after a brief talk, he dies peacefully.
As the battle between the Imperial and Alliance fleets continues, Lando leads a group of Rebel ships into the Death Star's core and destroys the main reactor. As Luke escapes on a shuttle with his father's body, the Falcon flies out of the Death Star as the station explodes. On Endor, Leia reveals to Han that Luke is her brother, and they kiss. Luke returns to Endor and cremates his father's body on a funeral pyre. As the Rebels celebrate their victory over the Empire, Luke smiles as he sees the ghosts of Obi-Wan, Yoda, and the redeemed Anakin watching over them.

Question: How is Luke related to Leia?
Answer: Brother