Tearmoon Empire: Volume 3 by Nozomu Mochitsuki

Half a year has passed since the Remno revolution ended. Spring has rolled around again, bringing with it what Mia considers to be one of the most important occasions of the year — spring break. Freed from the duties of school and station, she’s ready to seize the day! By which she means, of course, do absolutely nothing productive and lounge around in bed all day.

Her indolent plans are cut short, however, when her own granddaughter, Bel, is whisked backwards through time and deposited in front of her. Just when she thought she was finally done with worrying about the guillotine, she learns of the grisly future awaiting her, in which a war of succession rends the empire in two. Mia herself has been killed by poison, and Bel narrowly avoided execution by leaping through time.

As if that wasn’t enough, she discovers that this terrible chain of events can be traced back to Rafina’s tragic transformation into a dictator. Altering the course of history will, of all things, require her to defeat Rafina in the upcoming election for student council president!

Underprepared, polling abysmally, and trailed by a granddaughter who inherited all her, erm, Mia-est bits, she once again finds herself at a historical turning point... without the faintest clue of what she's supposed to do!