After the mysterious arrival of a stolen diamond and her missing brother's diary, an art forgery expert discovers that her long lost family was involved in an unsolved international crime, and is forced on a quest through the dark catacombs of Siberia to discover the truth

So many disconnects, as Nir pointed out. I'll add just two more.

How does and art fraud investigator's background relate to a jewel theft?

Why does she go to Europe when Siberia is in Asia? ?I had to google Siberian catacombs as I was only familiar with the European ones, Nice setting. Kudos for that but I not seeing the need for it in the logline.

Why 'must' or ?how 'thrust'? ?Doesn't look like anything compelling other than curiosity. As opposed to this thirty word version: ??When the?stolen Eye of the Czar diamond, ?a cryptic message. and the authorities land on her doorstep, an art fraud investigator must?prove she is not her father's?'fence'.

I only do exactly 30 word loglines (it's a personal challenge not some silly rule you need to follow). But it hints at a lot of the elements you've given us so far. Eye of the Czar ratchets up the 'importance' of the theft and links it to Siberia. ?Her father's fence is what the authorities believe but it leaves it open as to what his real guilt or innocence or involvement is. and now she has to go on her journey or risk going to jail. This may not be your story but I could see where it could make your first ten pages the page turner it needs to be.

Anyway, good luck with it. Take what you want from it. Trash the rest.

A fraud investigator must go to Europe and through Siberian catacombs to find out the truth about her father who mastermind an international criminal heist.

Thank you
Working to improve it,
Submitted another one - to have a choice from two options:

When an art fraud detective receives a package containing a stolen diamond along with a cryptic message from her dead brother, she is thrust on a journey to uncover the link between her family and an unsolved international crime