That is NOT true. What you stated. She was NEVER barren. This was malicious lie Keira's grandfather and the rest of Keira's mother family spread to defame the 1st wife.
Rowena the first wife was terrorised and harassed by the 2nd wife (Keira's mom) family.
They knew the Duke favoured her over 2nd wife since the Duke's marriage to keira's mother was something they had him forced into due to their political power and pressure.
They wanted Keira's mother to be the sole duchess and drive Rowena out of the Parvis estate. Rowena pleaded with the Duke that she was being targeted and harassed by 2nd wife family. He didn't take her concerns seriously and wouldn't intervene against 2nd wife family due to his "neutrality" policy.
The 2nd wife family (Keira's grandfather and Co) spread malicious lies about Rowena across the empire the main one being that she was "barren" to pressure the Duke into divorcing her.
Marquis Edinburgh not only targeted Rowenna but he ensured that she lost her status and any backing she would get from her family the weinbergs by having them completely wiped out by either having them killed or executed for crimes they didn't commit.
The count is the only survivor and wants revenge.
When Rowenna did get pregnant (apparently with fraternal twins) Keira's grandfather hired assassins to kill her and terminate her pregnancy because he believed it threatened his daughter's position or any future children she may have.
The assassination attempt is part successful Rowenna's babies dies and in despair and desperation she makes a contract with a demon to save her children in exchange for her life and get justice. She dies and so do the babies but the demon enters the baby girl dead body and thus the "Cosette" we see in story. Is a demon occupying the dead body of Keira's half sister that was killed by her grandfather.
apparently corset or whatever tf her name is like possessed by a devil. her mom made a deal with the devil to have a child basically cuz the duke neglected her some shit like that cuz she couldn’t bear a child. this plot twist is so unhinged to me cuz i never expected that.