Princess Leia secretly used the dark side of the Force when she killed Jabba the Hutt in Return of the Jedi. Leia Organa grew up with no idea she was the daughter of a Jedi Knight, or the Sith Lord he became. As influential as she may have been to the Rebellion, any feats she accomplished using the Force were entirely unconscious. And then, finally, in Return of the Jedi she learned she was the sister of Luke Skywalker.
The revelation turned Leia's life on its head. She was still an orphan, and she would always consider Alderaan – the planet she had been raised upon – as her lost homeworld. But now she was no longer quite so alone; she had a brother, as well as her beloved Han Solo, because the complicated relationship between those three was finally resolved. Unfortunately, the news also brought with it a dark legacy. Leia learned her father was actually Anakin Skywalker, who fell to the dark side and became Darth Vader.
The emotional impact of this discovery is explored in Claudia Gray's novel Bloodline. Set decades after Return of the Jedi, it sees Leia reflect back on how her life had been shaped by the Force. She came to recognize that the Force had always been with her, that it had subtly guided her all her life, and that there had been many occasions where she had unknowingly drawn upon it. Disturbingly, Leia realized she had actually done so while rescuing Han from Jabba the Hutt; she had channeled the dark side when she had attacked Jabba on his Sail Barge, using it to enhance her strength and allow her to choke the Hutt to death.
Luke was always concerned about the possibility he would fall to the dark side, but there is a sense in which Leia was the one most obviously vulnerable. Fierce and independent, Leia had a furious temper, one fueled by her anger at the injustices she saw in the galaxy. As righteous as Leia's anger may have been, anger is still of the dark side. The idea she had been unknowingly drawing on the dark side terrified Leia, because it suggested she was truly her father's daughter, and she feared following in Darth Vader's footsteps. In Bloodline, Leia was forced to admit to herself she had no idea why Anakin Skywalker fell to the dark side in the first place, and she dared wonder if he had done so for the very best of reasons – whether out of righteous anger or even out of love. She was not far from the truth.
This helps explain why Leia brought her Jedi training to an end. According to Star Wars: The Rise of Skywalker, she ultimately decided to Leia abandoned her Jedi training her lessons in the Force when she received a vision warning her that her son would fall to the dark side. No doubt she concluded the dark side was too strong in her family for her to safely train in the ways of the Force, and she would likewise presumably only have sent Ben to learn from her brother Luke when she felt she had no choice.
