This was explicitly not widespread knowledge for a long time in the Canon version of events. Luke, Han, and Leia (and R2-D2) kept this information to themselves following the end of Return of the Jedi, and didn't even tell Ben Solo, since they were planning to wait until he was older:
Before Ben was born, Leia made the difficult decision to shield her son from the truth of the monstrous Darth Vader's identity until the time was right—just as Bail Organa had kept the full story of Leia's own relation to the Sith Lord from her until she was mature enough to carry the burden.
(source: Skywalker: A Family at War, page 228)
However, Luke and Leia's parentage was eventually discovered and revealed to the public by Leia's political opponents approximately 24 years after Return of the Jedi:
Soon after, Leia's rivals ended her storied career in the political sphere by means of a cruel memento discovered inside her childhood hope chest from Alderaan, which had been in the safekeeping of an Organa relation on Birren. The Centrists unearthed a message that Bail Organa had recorded before his death, disclosing the truth of Leia's biological lineage.
The news that Leia Organa and Luke Skywalker were the offspring of Darth Vader was nearly as explosive as the recent bombing. Overnight, Leia went from an unimpeachable figure to a political pariah. Only a few kind friends remained loyal, risking their own career suicide.
(source: Skywalker: A Family at War, page 240)