Twilight’s plight had reminded her of the satisfaction she’d gotten from working Sunset through her issues. But if she’d been successful and Sunset had indeed worked through them, perhaps she really had outgrown what they had. The thought felt almost bittersweet until she glanced back down at the new victim of her whims and Rarity grinned wide.

Not gonna lie, that last sentence in this paragraph almost ruined it. "the new victim of her whims". Rarity comes across as extremely self centred and manipulative. She's forced herself and her fetishes / kinks on Twilight, who was given no choice until Rarity had effectively taken the choice away.

It makes it sound like Sunset had a lucky escape.

If that was the intention, then well done.

Someone needs to teach Rarity a lesson.