Well, I don't why you AREN'T making such a big deal out of it. It was something totally unexpected - those guys who seemed to have expected it, really?
I for one didn't expect it. To me, this Zelda game was a departure from the usual affair, being more of a rescue. If there are any plot twists and so forth in Wind Waker, they must be pretty drastic after I saw that picture with the 'two' spoilers.
I have been trying to keep myself as spoiler free from Wind Waker as possible. The sister issue isn't a spoiler, it's what's motivating Link to go on the adventure. I remember when Majora's Mask was nearly out. IGN64 had movies of possibly everything in the game, and I, having just bought computer with internet, downloaded every single one. That spoiled MM for me, and I have wanted to be as unknown as possible to the main story elements and developmentsof Wind Waker.