Story highlights Sibling superstars Sonny Bill and Niall Williams face off

(CNN) Sibling rivalry afflicts many households all over the world -- but not, it would seem, Sonny Bill Williams' family home.

Both he and his sister Niall are starring in the New Zealand rugby sevens teams and hoping to earn a much-coveted place in their respective squads for this year's Olympic Games.

So CNN decided to test the pair's affinity and to see just how well they know each other.

Question one: What are your nicknames?

Sonny Bill Williams: "SBW and Nizzle."

Niall Williams: "Nizzle and, the main one known around the world, SBW -- but just at home we call him Sonny."

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