that's just bad directingOriginally posted by bueno bob
It's not that he forgot about her or didn't know about her existance, it's just that she'd been out of the running for Luke's role until Yoda said otherwise.
Ben: "That boy is our last hope..."
Yoda: "No - there is another."
Scene ends. But afterwards:
Ben: "His sister? Should we attempt that, should he fail?"
Yoda: "No other choice will we have."
that's kind of key to the story so those extra two lines you've suggested should have been in there