"There's something familiar about this place." doesn't need to be clarified... he then says "Like something out of a dream or..." He probably had a vision of it in a dream...
I think it's clear Leia knew she was adopted. Luke says specifically "Do you remember your mother? ... Your REAL mother?". She'd have said something if she didn't know. It a plot hole. What's the bet in the next DVD release Lucas has removed that line "... your real mother?" and made that scene completely pointless?