In TESB after Luke leaves Dagobah and Obi-wan says “That boy is our last hope” and then Yoda says “No, there is another._.”, was he simply reminding Obi-wan about Leia? Or did Obi-wan not know about her being Lukes sister until ROTJ?
Real answer is that Leia wasn’t Luke’s sister until Georgie boy retconned it ROTJ.
Canonical answer is that Obi-wan kinda forgot about Leia being Luke’s sister after Episode 3.
My answer is that Obi-wan just didn’t know Luke had a sister, for whatever reason. Disregard Prequel Nonsense.