Well Alice says she cant see Jacob and she cant see Renesme either. And it could be because JAcob is a half breed half wolf(shapeshifter) an half man. And Renesme is half Vampire and half girl.? making any sense? im probably not explaning well lol
It's because of the chromosome count. Remember how in the book Dr. Cullen was talking about that? He said that while humans have 23, werewolves have 24 and vampires have 25 (or it may have been vamps 24 and werewolves 25 it's hard to recall now). Anywho, if I recall correctly Renesmeee has the same chromosome count as the werewolves. It's kind of like what someone else down there was saying, about it being related to being a hybrid.
On page 236: "I wish I could get a better idea of what exactly it is," Carlisle murmered... (And then Carlisle mentions that he probably can't do an amnio on it.) "The more I know about the fetus, the better I can estimate what it will be capable of... If I knew even the chromosomal count..." Then he asks Jacob how much biology he had taken, and then Jacob replied that he recalled we have 23 chromosomes, and then Carlisle said that vampires have 25 and on the next page tells Jacob that he has 24. Later in the book it mentions that Nessie has 24 chromosomes. If you want me to find it I will, but I have to go to class now.