Lena Thorul is added to the Superman mythos in Lois Lane #23 (Dec 1960). Lois meets Lena while investigating a story on witchcraft. She soon learns that mysterious things happen around Lena. Further investigation involves Superman and together they uncover that Luthor is behind the seemingly-supernatural incidences. Superman reveals the truth: Lena is Luthor’s sister.
Superman reveals that “Thorul” is an anagram of “Luthor”. In the DC universe, I guess this is enough to establish that one person is related to another (?!?). Lex admits and explains that his father disowned him when he turned to crime and changed their name to Thorul. Afterwards, a car accident made Lena an orphan.
Other than the introduction of a new recurring character into the universe, what is most interesting to me is the reveal that Luthor actually has a heart and wants to protect his sister from being exposed as a Luthor.
I can think of two other story moments in 1960 that the start of humanization of Lex Luthor: Lex Luthor’s origin as a misunderstood teen scientist and that moment when an adult Luthor expresses regret for accidentally foiling Superboy plans to prevent the assassination of Abraham Lincoln (Superboy #85).