She's not sueing the Stepmom, but the Dukedome itself. So, if things had gone by the book, Stepmom would have been locked up for a bit, and Bakadad would have to break out the chequebook and pay for the damages. Maybe even return some of the property that was under her mom.
I get the feeling, Aside from Mel's mom being Poisoned under pretext of "Curse"
My senses tell me that Two Dukedom killed 6 of 8 Imperial Siblings, and weakening their Royal Influences
To be honest, This not enough for this btch, this female dog need more suffer