As you might have guessed, this world is the world of an otome game. Magnolia (age 16) is a secondary villainess. She and the main villainess Gardenia (age 17) are competing for the crown princess position with the loser becoming a concubine. In the game her defining features were her ditziness and huge chest. She would harass the obligatory commoner/baron's illegitimate daughter heroine Margaret (age 16) while battling with the main villainess for the prince's love. As the trope goes, the heroine wins, both villainesses have the engagement broken off in a public condemnation event, and they are confined to a convent for the rest of their lives.
Adding insult to injury, Magnolia's father Gerald (age 42) and brother Brian (age 19) were both enthralled with the heroine and did not defend her as she cried out to them. However, Magnolia's bullying was so pathetically low-level it only endeared her to the fans of the game. Her character design (beautiful, dumb, huge tits) made her so popular, she became the heroine of the sequel.
The set up of that is as follows:
Prince Julius (age 18) studying from abroad at the academy alongside heroine, Magnolia, etc. could not bear to see her mistreatment and tried to ask for her hand in marriage. The request was denied because criminal scum was not appropriate for a foreign prince. Later on, Uncle Claude (age 32) hears what happened and is heartbroken that a niece he had never even heard of had gone through this. He protests that the punishment of life imprisonment did not fit the crime and asked to take Magnolia back to Azenda. This request is also denied.
In desperation, Claude seeks out Magnolia himself and manages to catch a glimpse of her in the carriage as she is led away. He falls in love with her at first sight. He reveals the secret of his birth as the Grand Duke of Azenda everyone believed to be dead. He declares the reestablishment of the Principality of Azenda, raises an army, and wages war against the Kingdom of Ascald to free Magnolia. Hearing this news, Prince Julius also declares war on Ascald to save Magnolia.
Gerald's reason for concealing Magnolia all this time is because he has seen this future. The best solution he could come up with is concealing her existence for as long as he can because the story is trying to correct everything he is doing back to the original plot.