Yolanda might very likely know the private affairs of the Princess Mary through Twonette, who was a friend of Her Highness.
"But you have not promised to visit Uncle Castleman's house when he invites you," said Yolanda, drawing Max again to the bench beside her.
"I gladly promise," said Max.
"That brings me to the third promise I desire," said Yolanda.

"I want you to give me your word that you will not leave Burgundy within one month from this day, unless I give you permission." "I cannot grant you that promise, Fraeulein," answered Max.
"Ah, but you must, you shall," cried Yolanda, desperately clutching his huge arms with her small hands and clinging to him.

"I will scream, I will waken the town.

I will not leave you, and you shall not shake me off till I have your promise.

I may not give you my reasons, but trust me, Max, trust me.