Hello,
I don't think your wife has a "mental illness." However, she does not seem like someone whose "all emotions are given to him" (OronD). Working from your own description of her emotional state (sitting "quiet and blank"), I would judge that she is in some kind of much less self-led or self-understood state. I don't know what to say since you didn't give much description. It seems like the two of you are very much unable to communicate; you are paralyzed by her lack of communication, and she is paralyzed by her relationship with this other person.
It is important for you to say more, or you will only solicit dire responses like that of OronD. Of course, he MAY be correct, but that would not be because of necessity, but only because of a perpetuation of the way everyone is relating at the moment. If nothing changes, then OronD is right, probably. But that is because there seems to be no, or little, self-conscious and differentiated behavior at play. The situation seems very depressed, and that is absolutely not a necessary place to be. It is an easy and a self-pitying place to be. Forgive me for saying so.
I wish you all the best in your rising, with your wife and children, to a very difficult emotional struggle.
Please take care.