Teenage Kara said she closely followed the Superman/Lex ordeal. It wouldn’t be much of a stretch to believe that Kara, at one point, came across photos/info of the infamous Luthor’s sister.
Which bring me to the scene where the two meet.
Kara looking at Lena like, “Oh, Rao. Gollygoshdarn, real life Lena is 1000x better than all of the images combined in my “Top 100 Lena Luthor” folder I’ve complied for research. (background music in Kara’s mind provided by Marvin Gay)