What could Ariana be up to?

If Tessa could, she would have gone on the journey. She would have done this no matter the risk because she knew her sister was not to be trusted.

Ariana is the most conniving person she knows but no one ever noticed it. She managed to fool everyone with her false sweet exterior. No one saw how bitter she was including her father but Tessa knew better.

"Why can't I open it?" her father asked.

"Because only I can"

"Why is that?"

"That's one of the rules of the book. The Sphinx explained that I will be the only one who can open it"...