Alec is amazing. I love him and Magnus together. But just to get this out of the way early...
I think he's going to die.
Why? Because CC did say that one of the Lightwoods would die, and she tends to go for more of a dramatic punch, and Alec dieing would definately be the most dramatic out of the Lightwoods. Can't see Isabelle so much, because she hasn't had much to prove in the story, unlike Alec, who has grown more as a character. Those are just my thoughts.
Now that's some sisky business.