Having to refer to a loved one by name when using voice commands isn't the worst thing in the world, but it can be annoying. Up until recently, saying "OK Google, call my brother" wasn't possible. But as of last week, Google has updated Google Search (and thus Google Now), adding the ability to recognize commands based on the relation the contact is to you.
But in order to use it, Google Now will need to learn a bit more about your contacts.
I'm using the relationship of brother for the purpose of this post, but you can assign contacts as your grandparents, mom, dad, siblings, significant other and so forth. Just replace "brother" with the respective relationship type for your purposes.
To set a relationship for a specific contact, you can use one of two methods. You can state, "Justin is my brother," and Google will confirm the selection using the contacts in your address book. If there are multiple contacts with the same name, you'll be asked to select the proper contact. The same process is required if the person has more than one phone number, and you'll be asked to select the phone number you want to use.
The second method is to give command, "Call my brother." After recognizing you have yet to set a contact as your brother, you'll be asked to select the contact whom you'd like to set as your brother.
It's that simple. Going forward, you can give the simple command, "Text/call my brother," and Google Now will carry out your wishes.
What's weird about the assignment of a relationship is that it doesn't show up in the person's contact card as it does with iOS. So if you wanted to remove a relationship you've set (perhaps after a breakup) you have to say something along the lines of "(Name) is not my wife."