It's a niche feature that has been there since at least iOS 4, (or at least I've known about it since iPhone 4, when I had the time to meticulously comb through every app on my Phone), but it wasn't actually useful until several interations of Siri later.
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.