I'd have preferred a different origin story overall. Neither the movie nor the series left me with one that was satisfying, but at least it doesn't bother me too much (save certain jarring moments). It wouldn't even have to be a lot. A single line or so (and I could see Buffy and Willow both curious) on how the Slayers knewn Kendra was a potential but not Buffy would be one example to slip something in.
Of course Kendra would only know what she was taught, and I don't consider the Watcher's Council a reliable narrator when it comes to the origin of Slayers. It's also possible (likely even) that the Council in its history ALTERED the process, much as Willow would later, which raises even more questions on the origin.
This could slowly come out in later seasons, perhaps as Willow even looked into how make Buffy more powerful to face Glory, to focus her powers, augmented by witchcraft, even if it was only temporary. It's not unprecedented. They did this when sending Buffy against Adam who apparently had powers over other dimensional energies in addition to being enhanced. Naturally, to avoid Sineya's wrath, Willow and Tara would look for something more specific (and might face the Council's menace or even wrath over it).
(I think it would be cool if the ring known as the Gem of Amara was one vampire sorcerer's attempt to counter the Slayer, that is "two can play at that magical game, and my way makes sure only the most worthy gets the powers instead of some random girl." Though the disadvantage sets in with other vampires gang up on him, and then start mass killing each other over the ring until its lost and becomes the "unholy grail of vampires."
That is, a weapon made by vampires for vampires to use against the Slayer turns out to slay more vampires as vampires kill each other for it than the Slayer herself!)
As trivial as it is, what bothers me is that when Sineya is first shown, she can't speak, she has to speak through another, but then in season 5 she can.