Complex sentence: Alice has theorized that she cannot see Renesmee and the "werewolves" because she has no experience as being one of them; she sees vampires because she is one, and can see humans "okay" because she was one.
Split into simpler sentences: Alice has theorized that she cannot see Renesmee and the "werewolves" because she has no experience as being one of them. She sees vampires because she is one, and can see humans "okay" because she was one.