If you really want to be able to emulate the characters in Apple's promotional videos, then you need to be able to tell Siri to "call mum" and "email dad." Thanks to the custom label field built into iCloud contacts, you can assign any kind of relationship to people you know. Tell Siri about your best friend, your boss, and your arch-enemy and you can control the virtual assistant more easily with your voice.
Activate Siri with a long press on the Home button to tell her (or him) about the relationships you have with your contacts: "Dave is my brother," "Amy is my mother-in-law," and so on. A confirmation screen appears where you can double-check the app has understood your command correctly and pick between similarly-named contacts when applicable.
If you want to throw some nicknames or pet names into the mix, then Siri can cope with these too. Everything from "my oldest friend" to "my editor" is saved in your own contact card, where you can edit and remove labels as required. You can even specify relationships between contacts other than yourself, like Steve's mum, etc. Just beware that if you're syncing contacts through Google Contacts or Microsoft Exchange, some of these custom labels may be lost. So just make sure you don't forget who your mum is.