Anybody can sue anybody for anything. The question is, do you have grounds to sue that won't get you laughed out of court.
Patents are supposed to cover implementations of inventions. Mere combination of existing ideas is not supposed to be covered. Watson and Siri (and Google Voice Search) combine voice recognition, natural language processing, and some sort of inference engine, but it's unlikely that any of them are using exactly the same techniques for all the parts.
