This is the first draft of a logline for this concept that is getting close to describing a causally connected plot. Well done!
Main character - art forgery expert
Inciting incident - discovers that her father was behind an unsolved international diamonds robbery
Causal connection - Because of finding out her father is accused of theft she must...
Goal - absolved her father
A few notes.
This line "...whose family disappeared 20 years ago,..." is unrelated to the plot and needs to be removed from the logline.
Main Character - art forgery expert - still a bit vague as I'm not sure this is an actually profession, is it?
Also how is an art expert able to solve an international crime? The main character description would be best used to describe either a characteristic as an obstacle that will stand in her way of achieving her goal and require she overcomes it via her inner journey, or, be an ability vital for her to achieve the goal. Currently art forgery expert doesn't do either.
Inciting incident - discovers father is guilty of a crime. Needs more detail, how does she discover? What was the event that sparked her suspicions?
On that note, what does "...behind an unsolved international diamonds robbery..." mean? Was he the robber? The master mind? The getaway driver? The dealer?
Goal - Lastly a logic flaw in the logline is; if he is indeed behind the robbery then he is guilty and she should not be able to absolve him. Secondly if the crime was unsolved then he would have not been accused of anything so in who's eyes is she trying to absolve him?