With the mysterious arrival of a stolen diamond, a forensic investigator goes on a quest to Siberia to uncover a decades-old international crime linked to her lost family

Great points above. Seems like an interesting idea, keep going with it!

The last draft of the logline still fails to create a clear connection between the crucial plot points. The event that starts her off on her journey (the inciting incident arrival of a diamond and journal) is not directly connected to the goal of her journey to Siberia (find the truth).

In what way is the arrival of a mysterious diamond and her brother's diary, an out of the ordinary and significant event that motivates her to need to do something?

Secondly what is the truth she must uncover in Siberia? What is her actual goal? What is it that the audience will want to see her achieve?

Lastly how is the diamond's and journal's arrival connected to the truth in Siberia? How do the former necessitate the later?

I get that her family is from Siberia, but maybe make that more clear?

Thank you:

After the mysterious arrival of a stolen diamond and her brother diary, an art forgery expert discovers that her long lost family was involved in unsolved and decades-old international crime, she goes on a quest to Siberia to find the truth.

I still have a problem seeing a clear and credible cause and effect relationship between the arrival of the diamond and the subsequent quest to Siberia. What is there about the arrival of the stolen diamond that triggers the character to go on the quest? It's 'mysterious' -- how can she deduce from an object shrouded in mystery that she's got to go to Siberia to solve an old crime?

And what's at stake in solving a decades old crime? What is the urgent import of a past event in the present life of the main character? Why must she solve the crime NOW -- or else. Or else what?

There is a slight tone difference in her going or being sent. You can take advantage of that. "Quest" can police are a little disjointed. How does the arrival of the diamond connect and trigger her actions?

This may seem a lot to achieve but it sells the story.