Sabriel suspects the orb is located in Vrefesea, hidden behind a combination lock somewhere in a property owned by Hortensia Day, and will try to find out if Hortensia owned a home or office there. Given the letter emphasized the siblings working together, as well as the words 'Think of your older brother' and 'think of your younger sister' and 'remember to take care of each other, all of you', Sabriel will try to reach any such property, and look for a lock, especially near or behind something associated with/depicting the siblings, particularly Luna and Oden, the youngest sister and and older brother, and try using the combination found in the letter.

She would absolutely share her theory with others, and suggest they go with her, or split up if there are multiple properties.