Google Now has more than one trick up its sleeve. If you're prepared to take a few minutes to set it up, you can have a card display whenever a family member arrives at home (or work). Here's how to get the feature activated on your device.
From the Google app menu tap Settings then Accounts & privacy and Commute sharing. As you can probably guess, you need to make sure the Let them see your commute updates option is on. For the time being the option to share your isn't available in the iOS app, though of course you can access Google Now cards inside the Google app.
Tap Choose who can see your location using Google+ (remember it?) to do exactly that. Here we're using a family group as an example, but you could just as easily switch it to a select group of friends, or your work colleagues, or just your significant other. Tap Change under the Pinpoint Location and you can pick Circles or people accordingly.
Google Now then uses what it knows about your home and work locations to broadcast your arrival at either place (you can change these locations via the Customise option on the Google app menu). Note that the rest of your family will need to have you in their Google+ Circles for this to work.