There is something fishy about the way you wont give up on your lunch buddy. Think of it like smoking. Sure we all have heard about the guy who smoked three packs a day and got run over by a bus at the ripe old age of 97 (cancer free). That guy is the urban legend. Reality is smoking may not kill everyone but it sure takes out its share, and those it doesnt kill it maims. So what is a smoker to do? Give it up before it is too late. The heavy smoker may not have cancer yet but odds arent in their favor.
You are the three pack a day guy, metaphorically speaking.You may not have crossed the actual line that would cause the wound to your marriage that may be terminal. But man youre still smoking. LET IT GO.
Dont let stupid pride get in the way.
And you dont have to make a thing about it. Just back off. Act busy, make other lunch plans, invite other coworkers, get on a crazy only tofu diet and tell her youre eating at your desk from now on...who cares. Just STOP IT.
Finally, this is not about your brother. This is all about YOU. I think you are trying to rationalize what your doing when you say your sensitive because of your brother. In acutality you feel guilty because your brother's wife probably started out innocently having lunch/conversation/email with a guy and now your brother is headed for divorce court. In the back of your mind Jiminey Cricket is saying, "Hey arent you doing the same thing?"
Listen to that voice. Listen to the people here.
Think about it. Why did you really come here? Most people dont understand what there doing until they are so stuck in the mire they are drowning. You have a chance. Someone is tossing you a life jacket. Put it on for goodness sake!
If lunch buddy is nothing to you and your wife and marriage mean something, distance yourself from this woman. NOW.(Or any woman for that matter)
If you still manage to rationalize to yourself why you should ignore all the warnings and procede, do as another poster suggested...give your wife the address to this site. She will need it.