If he is so obsessed with her, then he came too late to the party, if he wanted he should "saved" her before the wedding to the prime minister or at least some months after, she is being married for more than a year now.
(And I think he should have gotten in the way of the engagement of the prince and her is he likes her)
I think the perpetrator could be a girl obsessed with the brother and jealous of Celestine
You have a point but my other guess could be that he was busy and the last time he heard from her might be when he told him she was the prince fiance and was going to marry him and was the most happiest girl alive.. then after everything happened she might of remember she had a brother but a little too late and the brother was most likely waiting for a letter to be sent to head to her wedding.. remember the prince arrange the marriage without consent
So I’m guessing that might her older .. he might believe that her sister is being held hostage or manipulated ╮(￣▽￣)╭