This guy seems sweet and harmless enough, but why don't I want my sister to say yes to him?

[Guy With A Twin Sister]

My sister and I have an arrangement together, since we're in the same classes, that just one of us would buy the textbooks and one would borrow from the other. That way, our parents would only pay for one, not two (cheaper, right?).
So yesterday, I borrowed her textbook and as I was flipping through the pages for our reading, lo and behold, I found a slip of paper that said: "Dear Lydia, I've always believed in the kind of love Noah and Allie had for each other. I knew it was out there somewhere. But I didn't think it was for me. That I wasn't lucky enough. That others were luckier than I was. But I was wrong. Today, I just found the Allie to my Noah."

And the guy posts "We should hang out together sometime" on her wall/timeline on Facebook. And stuff like, "So what if others have a problem with your left eye being smaller than your right?! I don't. Screw them! You're perfect to me."
As the protective big brother, should I talk to the guy and tell him to back off? Do I let them be? Should I meet him, check him out, give him a chance then make a decision from there?
She better treat my twin sister right!
The guy seems like a nice guy, sweet from the looks of it, a total nerd who everyone ignores, and he is cute. But still, why do I feel threatened like he'll hurt my sister like a player? Is it just me being the older brother--that I want to protect my sister and question every guy that comes into her life?

[Dicky Bird]

Don't be over protective, give things a chance, why judge a book by it's cover ?

[PatrickC]

Back off. Your sister needs to make her own decisions. The shoe will be on the other foot someday and you need to respect her space. You certainly would want her to respect yours.

[Timmy]

Just talk to her about it and tell her your worried about her because you love her and that you don't really know the guy and are afraid he might hurt her