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I have no idea if this is the right answer or not but...Did you literally try telling Siri who your wife is? That's probably the first thing I would try.
 
I believe (don't have a 4S) that you just have to tell it, "My Wife is X," and then it will remember from then on.
 
You can either list her as your spouse on your contact card, or simply tell Siri who your wife is.
 
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You need to put yourself in you contacts. Add a field, spouse is a choice. Then go through settings to link your info to Siri
 
I just told Siri to text your wife and it went swimmingly. She'll be late for dinner tonight.
 
I just told it to text my wife and then it asked who my wife was. She was in my contacts so asked me if I wanted it to remember.

Pretty simple actually.
 
I asked Siri:

"Who is my wife?"

and Siri replied with a question:

"What is your wife's name?"

& then I associated it from there. Likewise for mother, father, etc.
 
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