It sounds like the two big problems with this concept are:

The translation of, the impact of the clues on the MC from book to film and a direct cause and effect relationship between the inciting incident and the goal.

In film the main plot points have to be visual. How does reading a diary visually explain that the MC is now motivated beyond doubt to take action? As inciting incidents go this is weak, is there another way the MC can be made to need to achieve her goal?

As previously mentioned no need to stick to the book page for page. When adapting a book you have to make concessions for film as you are writing for a different medium.

Secondly for the plot to make sense the plot points need to have a logical connection that necessitate each other. From the logline (and as DPG said) how does reading a diary make someone want, nay, need to go to Siberia?
Currently it doesn’t, so what is the connection between the brother and Siberia? Was he an archaeologist or treasure hunter who worked there?

Lastly don’t be afraid to elaborate on the plot in specific detail at these early stages of development. You are not at risk of giving away the plot when you are still trying to structure it for film. Description such as “…solve a riddle of decades old crime…” are too vague for a logline as they do not describe what happened.

Was the crime a murder? If so who was murdered? Was it a theft? If so what was stolen?

Biggest problem of the concept and logline is the goal.
Her solving the decades old crime is not part of the plot and potentially should not be in the logline. The goal is confusing, is her goal to find the criminal who did the crime or find the long lost family or find the treasure?

It can’t be all three, pick one main goal for the MC to pursue and mention only that in the logline. This will help tremendously when you come to structure the plot for the film. The other goals can be subplots or objectives for her to achieve throughout the story on her way to the main goal.

Hope this helps.

The latest draft of the logline is even more vague than the original. The use of descriptions such as “…promises to solve an old crime…” and “…secret of her vanished family..” do no elaborate on specific plot details.

Best to specify in a logline what the characters will actually do.

Secondly there are a few fundamental problems with the logline. The first is if the main character is made to go on a journey involuntarily then it is not interesting but if they have a burning need to achieve a goal and go on their journey motivated beyond doubt then they are worthy of an audience’s interest.
The second is there is no plot described because the logline lacks an inciting incident and a goal. Why must the main character go on their journey now and not a year ago or a year from today? What set the main character off on the journey? What is it the main character actually want and need to achieve? What is their goal? what will the end of the journey look like?

Don’t feel compelled to stick to the book page for page adapt it to the screen by converting the emotional charge of each chapter into cinematic visuals. Motivate the characters to act out of motivation and make choices that produce the same meanings as the inner monologue insights written in the book.

Hope this helps.