She thinks it’s Leia anyway even without knowing she’s his sister because of them communicating with the force at the end.
This is what everyone I’ve shown the movies to came away with too. A couple of em even called it when the communication thing happened. "OOH! So she’s gotta be “the other”, right?"
That she’s also Luke’s sister was a groaner almost every time. I’m interested to find out how she takes that one.