First of all, aside from the fact that Luke was told that Leia was his sister, how would he possibly know that Leia was adopted if she didn't know? Secondly, if Luke put such emphasis on the word real, would that not have garnered some kind of response from Leia if she had no idea what he was talking about? If she was under the impression she had only one mother, wouldn't the need to specify real (as opposed to not real) confuse her just a little bit?
First of all, aside from the fact that Luke was told that Leia was his sister, how would he possibly know that Leia was adopted if she didn't know? Secondly, if Luke put such emphasis on the word real, would that not have garnered some kind of response from Leia if she had no idea what he was talking about? If she was under the impression she had only one mother, wouldn't the need to specify real (as opposed to not real) confuse her just a little bit?