As the older brother, how should I react to this?

[Lloyd Da Vess]

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 this afternoon, 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!