Is my sister making him seem like he's really into me?

well , first off I met this guy , and he's really nice and super sweet , and we've already been going out for five months now . we broke up a little while ago - got back together , anyways , my sister says that she's planning something special with him . and so naturally she wont tell me . so then I go onto her facebook ( I know very wrong , I suppose the saying "curiosity kills the cat" fits nicely here) but when I looked at their messages together , she brought up the idea about the special thing (which is just suprising me by coming over ) then he didnt seem very enthusiastic about it . all he said was "hahaha , yeah that seems like a good idea " . then later when I asked her about it and asked "did he come up with it or did you " then she said "well I told him about it but then he said , 'oh my gosh! that's exactly what I was thinking!" .

and I wasnt about to be like I know youre lying . so , yess , what should I do about it ?

We don't just hear/read words as they are. We often reinterpret them based on our experiences and paraphrase (we're not perfect recorders. either). This could be misinterpretation/miscommunication, she could have spoken to him on text or something so you could only have half the story... or she's got something up her sleeve, although I don't yet see any motivation for that.

So, I'd say, don't let on yet. Don't get hopes up or down. Just watch the situation, gather more information, and respond when you have more information... that is IF a response is necessary.

And..... You're right. You totally invaded her privacy by logging into her fb. She must have trusted you to give you the pw, and you betrayed that trust. Not cool. Think of how violated you would feel if someone did that to you.