Wait, if those sisters were the last to have powers why don't more people know about resonance in that family? You'd think they would have also read her diary since they kept it. And no one tried to study the last two known spirit users and find out why they are the last or anything ekse? Why didn't she finish reading the diary? I think she should read it to the end, what if one sister lost her powers or something else terrible happened?
And how did Cosette find out she needed to imbue her sister in order for her powers to awaken. If she's also a spirit user then the future prophecy would still be correct, so nothing bad happened to the lands? So on that case did the garbage father only go back in time to save his dukedom and his other daughter is the one who would be able to stop Cosette revenge?