The sister in question usually becomes wicked-hot during puberty in a ugly-duckling to swan transformation, while the brother goes through a ugly-duckling to ugly-duck trnasformation.
Guy2: "OMFG bro, how the hell did Eugene and Liz come from the same vagina? Liz is the hottest piece of ass since Megan Fox."
Guy1: "Dude, I know! But according to the Anderspn-Best Thereom, every nerdy fag has a mega-hot sister."
Guy2: "Thank God for the Anderson-Best Thereom!" *high fives all around*