The Westermarck affect tends not to work for siblings who weren't children at the same time or for siblings who, for some reason, never met until adolescence or later.
Example 1: You're a male of age 12 or so when your sister is born. By the time you're 22, you have to strangle the internal Pedo-bear to death every time you see her, but she only sees you as an older brother.
Example 2: Your and your sister are only a couple years apart, but your parents divorced when you were very small children and one took each of you. You remember your sister as an infant. When you meet again as adolescents, you're both fairly likely to be attracted to each other.http://en.wikipedia.org/wiki/Genetic_sexual_attraction