Sounded like a good detective mystery, but the writer really dropped the ball. The trail of evidence leads plainly to the stepbrother as thief, although it is never explained why the head of the police seems determined to charge the professor with the theft, even though the detective directly investigating the case makes it plain that the professor is innocent.
The obvious twist should have been that the head of the police was the thief, with or without the collusion of the stepbrother. The wife of the professor could have been secretly involved as an accomplice of the head of the police - perhaps as his lover. This would have made the story much better, in my opinion.
I won't be listening to this one again.