I feel like there are tons of movies and stories are out there where our hero has to confront the villain and be tempted to join their side and they’re not related to the villain.
Yes, but this is a different situation. The film is giving Rey as a character literally everything she could ever want on a silver platter leading up to her confrontation with Palpatine. Leia is revealed to support Rey beyond all measure, Luke is telling her that a thousand generations of Jedi live in her now, etc. Legitimately the only thing that could ever tempt her to even consider Palpatine’s request is if she believed it were her fate, or her very nature, to give in to his demands. Ascendant already alleviates this quite a bit by giving her a vision of just her on the throne. The problem with her being the “granddaughter” of Palpatine is that we already know you can be the offspring of an evil person and still be a fantastic person in life, namely Leia in new canon. That’s why this is such a good idea. If Rey were manufactured using Sith alchemy and cloning specifically to be a suitable vessel for Palpatine, her confrontation with him quite literally defines why she was made in the first place. It’s much more harsh and uncomfortable to have to deal with - especially after being told that her anger towards Palpatine for killing her loving, adoptive parents means she can’t hope to defeat him without being possessed. At that moment, she begins to doubt Luke and Leia’s guidance, no matter how thoughtful it was, that perhaps her vision of the future was true and she must conform to her destiny.