Google Now finally lets you refer to people by relationship
Also, the system isn't quite as smart as you may want it to be. It falls prey to similar issues that you would have with Siri in this regard. For example, if you have someone set as your "mom", you can refer to that person also as "mother", but it won't work if you say "mommy" (or "mummy" depending on your accent). If you say "call mommy" it will claim to not know who that is and ask you to set that relationship every time, but the person you pick will be accessible by saying "call mom". So, certain terms still don't work right, but at least you can set it up for multiple same sex parents or siblings. Additionally, even shortened terms don't really work, like saying "call my bro" or "call my sis". In those cases, we found that Google Now will either think bro means dad and sis means mom, or it will ask you who you meant, but not assign that person to the term.
So, just be aware of those issues and you should be all good. Anyone with Google Now should be able to use this feature right now, because this looks like a server-side update, not an update to your Search app.
source: +Google
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Things that are NOT allowed: