Hello,
I’m still persuaded that the word “mysterious” should be avoided in a logline.

I don’t understant what is a art forgery expert so you have to find a better expression.

Then,
“After the mysterious arrival of a stolen diamond and her missing brother’s diary”
This is not interesting. Sorry.
What is interesting is what actually puts the story in motion and we must understand how and why.
A serial killer kidnap my son. I care, this starts a story.
I receive a diamond in my mailbox. Good for me. Then I go to Siberia looking for my missing family… Whaaaat!? Why!? This is not clear! Why? Logline Alert! You must come up with something that make sense, clearly, for everyone.
If the book is a good book, you must find what is the very main element that put the story in motion: it’s not a diamond, it’s not a diary. And it’s not the world “mysterious”.
Analyze the book. You must found something. A single thing, very very precise.

Then,
what is this thing about her family? Are they dead? Are they “missing people” (after a certain number of years this is legally equivalent to death)? Say things clearly. I don’t need to know if later in the movie we learn that they are not what we believed.

I give it a try…

When an anti-fraud art expert discovers that the disappearing of her family was linked to an art fraude, she ….. then what happens ?
Many things happens in the novel but you have to find what is the VERY MAIN confrontation. Siberian criminals ? Read the novel again and find the MAIN thing. There must be something. One thing.

And finally. Decide what precisely is her goal. Find her family? Solve as art forgery case?

You must understand the difference betwen teasing and writing a logline. You tease too much. In a logline you must build a solid structure.

Good luck.