For one thing, Ashley Biden is Joe Biden's daughter; not his niece.
... and for another thing, the linked article described the diary as having been "abandoned." That is a lie. Ashley Biden had stored several of her personal belongings in a friend's house. This house was later rented to the defendants. The defendants had no right to take possession of those items for the purpose of trying to sell them.
Duo plead guilty to plot to sell Biden daughter’s stolen diary to Project Veritas
The theft also involved much more than just a diary. Ashley Biden had also stored tax records and digital devices in her friend's house as well.
The defendants in this case sold Ashley Biden's property for $40,000 and even had the audacity to return to the houase after they had moved out, in order to collect more of Ashley Biden's belongings when they were asked to do so by members of Project Veritas.