Corrected entry: In Return of the Jedi, both Luke Skywalker and Obi-Wan Kenobi were not aware that there was another Skywalker, Leia, until Yoda explained to them. In Revenge of the Sith, it was very obvious that Obi-Wan was holding Luke and Leia was born while he was still nearby. Therefore the fact Obi-Wan did not know about Leia is somewhat strange.
Xofer
19th May 2005
Star Wars: Episode III - Revenge of the Sith (2005)
Correction: Obi-Wan, of course, knows Leia exists, as he had an eventful encounter with her when she was a child, shown in the Obi-Wan Kenobi miniseries detailing his bond with her, which would then tie in with what she describes in A New Hope about him serving her father (Bail Organa) during the Clone Wars.
Correction: Obi-Wan DID know about Leia (after all, he was the one who explained to Luke that he had a twin sister); in Empire, he mentions that Luke is their "last hope" because Leia doesn't know about her powers or heritage, and he doesn't think they'll be able train her in time (if at all).
Xofer