Hello! Obi-Wan was there at the birth, he should have known this.
Good point. It may have been that Obi-Wan was so focused on Luke becoming a Jedi that he forgotten that Leah would also have her father's prowess with the force. Yoda's comment simply put his view back into focus. But I haven't watched ESB in a while so I don't remember Obi-Wan's reaction to that comment. Maybe it was one of surpise like - Really? Another? Who is it? - or maybe it was one of - Oh, yeah... Leah... She completely skipped my mind! : )
Thinking back, the only scene I recall that Obi-Wan and Leah shared was in Episode IV wherein Obi-Wan is watching a hologram of Leah.
Regarding that... obviously, Obi-Wan does know of Leia's exsitence. He sees no hope in her, though, because she is a politician (Senator). Remember his deep dislike and mistrust of all politicians from Ep. II. Yoda disagrees, I guess.