The will of a non-Muslim for a Muslim is valid
20-3-2011 | IslamWeb
Question:
Assalaamu ‘Alaykum; Please calculate the inheritance according to the following information ۞- The will which the deceased left behind and that is related to his inheritance is : The deceased was christian but he willed all his fortune to his youngest brother (christian), youngest sister (converted muslim) and the youngest nephew (christian). All his money was divided equally among all three of the above. Is it acceptable for the muslim sister to receive this part of the will?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and Messenger.
First of all, we would say that the will of this non-Muslim deceased bequeathing his entire wealth is a will that exceeds one-third of his inheritance, so if he has heirs, it is only one-third of the will that should be approved, and whatever exceeds one-third depends on the consent of the heirs: they may approve of it if they wish or they may return it to the inheritance if they wish, exactly like the will of a Muslim.
Ash-Shaafi’i 
said in “Al-‘Umm”: “If a Christian left a will bequeathing more than a third (of his inheritance) and his heirs came to us (complaining), then we would invalidate what exceeds one-third (of the inheritance) if they so wish, in the same way that we would invalidate it if the heirs wish so in case the deceased was a Muslim."
Besides, Ibn Qudaamah said: “His [non-Muslim’s] will is valid only in the same way that the Muslim’s will is valid for another Muslim, so if he makes a will to his heir, or for a non-heir bequeathing more than one-third (of the inheritance), then this depends on the approval of the heirs, exactly like the case of the Muslim.”
On the other hand, if the will is ruled effective, then that Muslim sister is permitted to take what her brother willed to her, and she takes it as a will and not as inheritance, as the jurists stated that the will of a non-Muslim for a Muslim is valid. Zakariyyah Al-Ansaari from the Shaafi’i School of jurisprudence, said: “The will of a non-Muslim is valid, whether he had made it for a Muslim or a Thimmi (a non-Muslim living under Muslim rule)".
Allaah Knows best.