The spoilers didn't talk much about the previous instance of the younger sister and the red-haired older sister, so I'm gonna skip that here.
So basically, Cosette's mother was the first wife but because of a misunderstanding, she was driven out because her husband is a strict lawful neutral. (Dude, you're really putting a bad name on us lawful neutrals.)
Kiera's mother is the second wife, whose family then sent assassins after the first wife, just to be safe. They were sent to ensure that the first wife couldn't give birth to the next Saintess, so that's why the Grandpa looks like he's hiding something.
The assassins probably failed since Cosette's mother was able to give birth, although to a stillborn. Grieving, Cosette's mother makes a deal with a demon to inhabit Cosette's body. This is what gives Cosette the ability to steal Kiera's powers before she can manifest them. The "Resonance" is when Cosette gives Kiera's powers back as a final laugh.
Okay, so there's some debate about Cosette's real gender since some people say that Cosette was actually born a boy and only became a girl because the demon transformed the body into a girl's.
I think the boy argument makes more sense because that would invalidate any claim the Cosette had to be a Saintess, still making Kiera the eldest daughter.
But we don't know how the Saintess power is inherited that well, so I guess the girl claim works out too. Girl Cosette dying as a stillborn may invalidate her claim to the Saintess powers since she had no chance to live anyways, passing the power on the current eldest living daughter, Kiera. We know that if you kill the Saintess, her powers don't get inherited, so maybe if she dies from unavoidable means such as being stillborn, it doesn't count?
Either way, Kiera gets the Saintess powers because the real Cosette was stillborn. The only reason why Kiera can't manifest her powers is that a demon inhabited Cosette's body and stole the Saint powers from Kiera.