After your father (the Duke )dies you get know that your stepmother is a slave that your father bought and made his second wife. Your father forgot to release your stepmom from slavery. As the firstborn you are to be inherit everything your father owned . Fing de deta retaileates so that her daughter can get some property too but as she was the slave of the Duke her daughter is considered as a slave too . So the you get to keep your stepmother and stepsister as slave...