As Google doesn’t know who your mother or wife is, you will have to associate a contact to a relation the first time you are using a voice action to call or send text to any relative.
Google explains:
Want to save time keeping in touch with your loved ones? Or just not drop your phone or your shopping bags whilst you try to find a number to make a call? You can skip scrolling through your contacts and just say to your Android phone “Ok Google, call Mom” or “Ok Google, send a text to my wife.” Starting now, the Google search app on your Android phone will be able to pull up the right contact for you—and if you don’t have Mom saved as “mom,” it’ll help you set that up quickly.
The feature seems to be US-only for now as we have tried multiple times and it doesn’t work for us. If it works for you outside US, do let us know in comments.

2 replies on “Google Voice Search now allows you to make calls based on relationships”
Hello.
just wanted to let you know it is working for me. It was very easy to set up also 🙂
It works perfect for me (India) . First you have to check the Contact recognition under Google Now — > Settings –> Account & Privacy! 🙂