First:
Inheritance is a right due to heirs after the death of the deceased, and Islamic Sharī῾ah has regulated this matter by assigning each heir their specific share—whether individually or jointly with others.

Islamic Sharī῾ah restricts the authority of the deceased over their wealth after death. It does not permit altering the fixed shares of inheritance. For example, it prohibits making a waṣiyyah (bequest) to any of the heirs, as stated in the ḥadīth: “There is no bequest for an heir.” [Recorded by Abū Dāwūd and al-Tirmidhī from Abū Umāmah].

Islamic Sharī῾ah also sets the limit for what may be bequeathed. It allows a bequest of up to one-third of the estate to a non-heir without requiring the approval of the heirs. Anything beyond that percentage requires their consent.

Second:
A Muslim residing in non-Muslim countries (such as in the West) is obligated to arrange financial matters in accordance with Islamic Sharī῾ah—this includes settling debts and organizing inheritance via a legally recognized bequest—if they know that local laws contradict Islamic rulings.

If one knowingly fails to do so and is content with the local, non-Islamic inheritance laws, then they are sinful.

However, if they fail to do so due to negligence or oversight, then they are not sinful, but those who receive the wealth are required to distribute it according to Islamic Sharī῾ah.

Therefore, it is obligatory for a Muslim to name the rightful heirs and their shares in accordance with Islamic rulings and to document this in a way that is legally valid in their country.

If they were capable of doing this but did not, they bear sin for neglecting a financial obligation. It then becomes binding upon those who inherit the estate to carry out the division in accordance with Islamic Sharī῾ah.

Fatwā issued by Dr. Khālid Naṣr