See... sometimes you need to deal with a problem upfront to solve it permanently.
The sister stayed no more than a grudge seed, and didn't change one bit... just an arrogant, entitled asshole even after 3 centuries.
On the other hand, Dara-san, from the time she was alive, had done the hard work and dedication to be loved and grow from and with it.
In a way, this was the last push necessary for her to break away from that name that bonded her to that agonising "duty".
By the way, when we learned of Dara-san's original name last chapter, I went back a few chapters, because that name sounded familiar, and we had seen this name before, go to chapter 26, page 14, in the middle panel, her name as an enshrined goddess in the temple (sorry I don't know how to edit the image to pick just the right panel).
Anyway... now they have excised the impurity... shame she didn't burn it in holy fire too, but it is her decision.