I guess most people readily accepted Lori as being Jazz & Spaz' sister not only from the blurbs on the boxes but also to establish a sort of weird rule that within this series of games siblings would pop up at any moment.
Jazz 2 gave us Spaz. Based upon a design for a background prop in one of the Holiday Hare episodes he suddenly turned out to be a brother in the second game.
Jazz TSF gave us Lori. Based upon the design of a throw away character from the ending cinematic of Jazz 2 she suddenly turned out to be a sister in the expansion.
If anything this tradition should continue ultimately leading to 4 player co-op in the next game with an entire family of Jackrabbits. Though technically Eva would count as I'd assume she and Jazz finally got the deal done and got married.
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