You obviously haven't actually played that fight with Wakka's ultimate weapon. The counter hits Sin.
Fair enough on the Voodoo point, though.
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Sin was originally a mass of pyrafly's compressed and held together with gravity spells by Yu Yevon. given that once a aeon of anykind is possed by Yu Yevon it more or less just becomes a mass of pyraflys and therefore negates it status as an aeon. at least that's the impression i was given.
also the comment in operation Mi'hen about "sin always returns for it's spawn" seems to indicate it can absorb more masses of pyrafly's into itself, what would stop it from just absorbing the tonberry into itself, after all fiends are just a mass of pyraflys.