Here are some great tips for setting up nicknames, relations, work locations and more with Siri from Gigaom:
“You can put your personal relationships on record with Siri through your contact info, too. Add a new field to your contact info and select ‘Related People.’ When you tap on the label, you can choose from several different relationships. If that contact also happens to be in your Find My Friends list, you can simply ask Siri where your Wife, Husband, Mother, etc. is located. You can also tell Siri how contacts are related to you to automatically add this data to their Contact card.”
There’s a lot more, and I suggest you read the whole thing. Incidentally, I found a quick way to let Siri know who my wife is. I told it, “call my wife.” It responded by asking who my wife is, and I told it. It then asked, “Would you like me to remember that [wife’s name] is your wife?” “Yes,” I said, and since then, I can use the variable “my wife” and her actual name interchangeably with no problem.