> It's a niche feature that has been there since at least iOS 4
FWIW it's been in the MAC address book since the original OS X (jag-wire?)
Yes, the feature has been there since iOS 1.0. Siri on iOS 4.0 started making use of it to enable voice commands (e.g. “call Mom”)
Relationships is one of the features, to me that feels like Apple ended up building functionality on top of years after the feature was released, but since it was useless for so long nobody ever uses it.
Did you know not only could you specify your relationships, but other people's relationship's as well? I'm not sure if Siri lets you contact people this way ("Call John's mom"), but I wouldn't be surprised if a hidden update to Siri enabled this feature as well.