Is Zakat obligatory on an amount of 46,660 Saudi Riyals obtained from a father's inheritance after twenty years, knowing that some heirs caused difficulties and procrastination in handing over the inheritance?

If it is proven that the heirs killed their inheritor, then they have no right to the inheritance Islamically. The money you received after it was withheld for 20 years, according to the majority of scholars, its zakat is due for all the past years. According to the Maliki school and Ibn Uthaymeen, it is due for one year. And according to the Hanafis, you begin a new lunar year with it. The opinion of the majority is more cautious and absolves one's responsibility. The method for paying zakat for past years is explained in Fatwa no. 98644. There is no harm in following the opinion of a trustworthy scholar.