It wasn't so much that Leia didn't want to tell Han that Luke was her brother. It was more so that Leia, when Han brusquely confronts her, was awash with new revelations. She'd just discovered she has a brother, a twin no less.
Why did Leia kiss Luke if she knew they were siblings?
Following the Wampa attack during the movie's opening scenes on Hoth; Leia, Han, and Chewie visit Luke in the infirmary. Leia is annoyed with Han, who insists she has feelings for him, so she kisses Luke to prove she doesn't. Presumably, the scene was simply meant to make Han jealous.
When was it decided that Luke and Leia were siblings?
A New Hope set up the idea of them being attracted to each other before The Empire Strikes Back planted the seeds for their familial connection, which Episode VI ultimately confirmed. The fact that making Leia and Luke related wasn't originally planned is well-known, but the retcon's background is more obscure.
Darth Vader only learned he had a daughter in Return of the Jedi - but there's no evidence he realized she was Leia before his death and redemption. Darth Vader may have never known Princess Leia was his daughter in Star Wars and never cultivated a relationship with her, even when he was a Force ghost.
Star Wars VI: Return of the Jedi - "He is my brother" (Luke and Leia, Love Theme) (sub ITA)
How did Vader know Luke was his son but not Leia?
Vader was hunting down the pilot who blew up the Death Star, and hired Boba Fett to find them. Boba was unable to, but he did give Vader a name: “Skywalker.” This, combined with his knowing Luke was Force-sensitive and had been trained by Obi-Wan, was what made him realize the truth.
It is revealed that Ren is the son of Han and Leia, originally named Ben, and was once one of Luke's Jedi pupils. He was corrupted to the dark side of the Force by the First Order's Supreme Leader Snoke (Andy Serkis), and helped destroy Luke's new Jedi Academy.
Despite his mother being Padmé Amidala, a former queen and galactic hero, there is little mention in the Star Wars saga of Luke Skywalker learning about her life, or even being interested in finding out.
Alderaan's princess knew she had been adopted by Bail and Breha Organa after the Clone Wars, but she had no idea who her parents were. That was until Luke Skywalker told her the truth - that he was the son of Darth Vader, and that she was his sister.
Leia's Force abilities and her lack of awareness of her true identity also contributed to Vader's inability to sense their familial connection. Vader's denial of Anakin Skywalker's legacy and his fear of getting hurt again prevented him from realizing the truth about his children until later in the original trilogy.
As stated earlier, it was the force, the force grants great intuition. Remember that when Luke told Leia, even she said that she somehow always knew that she was his sister. And Leia had not even been trained in the ways of the force yet.
Following his nearly fatal encounter with a wampa on Hoth in The Empire Strikes Back, Luke Skywalker is rescued by Han Solo. In a medical center shortly after, Princess Leia checks on a recovering Luke, and in an effort to make Han jealous, she kisses the bedridden hero and walks away.
In the 2015 film Star Wars: Episode VII – The Force Awakens, it is revealed that Han had a son named Ben with Leia. The family breaks apart following Ben's descent into the dark side as Kylo Ren, with Leia and Han separating. Luke is not revealed to have any children.
These are family-friendly movies that will never make overly direct references to sex (much less show it take place). There is no implication or indication that Luke and Leia ever slept with one another, even before they knew they were siblings.
It's been established they are not twins. Ren is 10yrs older than Rey. Leia might be a badass, but no woman has that long of a labor! How deep is Rey and Kylo's force connection and how do they have one?
Han Solo marries Princess Leia and they have three children: twins named Jacen and Jaina, and a younger son named Anakin. Luke reestablishes the Jedi Order, marries a non-movie (but fan favorite) redheaded Force-wielder named Mara Jade, and they have a son, Ben Skywalker (named in honor of Obi-Wan “Ben” Kenobi).
This mystery was especially prevalent in Star Wars: The Force Awakens, but The Last Jedi overcorrected it by claiming Rey is not related to any established original trilogy Star Wars characters. Then The Rise of Skywalker revealed, in an out-of-left-field retcon, that Rey is the granddaughter of Emperor Palpatine.
She figured out her mother's identity after Return of the Jedi, when she learned she was the daughter of Anakin Skywalker; it didn't take Leia long to connect the dots between Anakin and the senator of Naboo he had spent so much time with during the Clone Wars, who died under mysterious circumstances on the day Leia ...
In Star Wars Legends Leia was visited by the force ghost of her father Anakin Skywalker, the man who used to be Darth Vader. It wasn't a pleasant meeting as Leia was unable to forgive him for the things he'd done no matter how hard he begged, and it was the only time she saw him.
Kylo develops a close relationship with Rey and starts feeling genuine affection towards her, so when Snoke reveals that he had manipulated their bond and tortures her in front of Ren, he looks furious at his master. Thus, when Snoke orders Ren to kill Rey, he chooses to save her and kill his master instead.
But if Palpatine essentially fathered Anakin through the Force, the Emperor would be the great-grandfather of Kylo Ren and the grandfather of Rey, making the pair first cousins once removed.
Having revealed that he used to be a Jedi Padawan under Luke Skywalker, Ben Solo changed his name to Kylo Ren in order to escape his lineage after turning to the dark side.
