Gossip Girl star Chace Crawford is dating country singer and American Idol alum Carrie Underwood. This is Carrie Underwood; and this is Chace's sister, Candice. She was just crowned Miss Missouri. [People, Perez Hilton]
I don't think its that weird, since a lot of times we're attracted to people that look like ourselves, and often siblings resemble each other. So its not really that he's looking for a girl who looks like his sister, but he's used to bland blondes as typically "normal" and "pretty."