Earlier, after Adric read the letter sent by Madame Rosalie, Oriana was back at the palace, and this time she was getting married. From what he read in the letter, the girl wasn't aware of the marriage, as it was arranged by her family. It was suspicious because the Crown Princess was still unmarried; if a pact was to be made, it always started first with the first daughter. Seeing that they picked Oriana only meant that her sister rejected the marriage.
And most importantly, Oriana's life was at risk. By societal construct, a maiden, except one who is a courtesan or openly serves men, isn't supposed to be seen or touched by another man except her husband, and any maiden who goes against this shall be stoned to death or, worse, decapitated. This was set to ensure that women had self-control and kept their modesty for one man.
"From my insider source in the palace, it says she's getting married to the Kingdom of Valonia," Madame Rosalie said in her letter.