Something being explainable and understandable doesn't mean the story is trying to justify it.
Whether or not Sakine forgives Hibiki is her prerogative. Sakine being head over heels for Chiyoko....well, the whole thing is kinda tangled up, which is the draw, of course.
Interesting that Sakine has two people after her, with only one (third) person in her sights. Chiyoko being the "lens" character of the series, and not having every girl (currently) after her, is the intriguing bit to me. The triangle of Chiyoko->Mafuyu / Mafuyu->Sakine / Sakine->Chiyoko is simple enough, but throwing Hibiki into the mix who's after Sakine and has always been hellbent on "exposing" what she believed to be the lie of love, is what throws it all into chaos and pushes the obvious/"formulaic" solutions out the window--even with there being 4 characters for the conventional pair-up ending.
Sakine has every reason to hate Hibiki. I try not to throw that word around too much, but Hibiki ruined her life, meant to do it, and then blamed Sakine for not "keeping to her word". She's even given up on any sort of "good ending" for herself here, and I kinda wonder whether the author's going to give it to us, as--realistically--I could see Chiyoko trying to remain friends with her, with Mafuyu simply not caring and Sakine wanting to write Hibiki out of her life. Anything that sees a mending of Hibiki & Sakine's relationship would take dozens of chapters of effort, or it would look cheap and contrived, and even then I wouldn't believe it.
It makes any "happy ending" for Hibiki hard to guess at. And in a way, a "happy ending" might be her confessing her sins and then serving penance by disappearing. Which, sounds bad, but Hibiki has to unlearn and then re-develop most of her life's worth of emotional growth and regulation. She's deeply troubled, in a way that a couple high school girls isn't going to magically solve. She recognizes it and is no longer running from it, and that's the first step, but....yeah.
Very dramatic, very toxic, not exactly novel, but I dig the interpersonal stuff like this when it can be viewed from a distance, and I do like seeing how authors nagivate these sorts of scenarios. So I'm most looking forward to how it shakes out, and whether Hibiki finds peace, and what that might look like.
Thanks for the TL.