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. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Your father-in-law's will to his son is a will for an heir and the Prophet said: ''Allaah has given each one his right and there is no will for an heir.'' [Ahmad] So, a will for an heir cannot be implemented except with the permission of other heirs. Your mother-in-law has the right in the house and other inheritance that her deceased husband left; she is entitled to one-eighth of the inheritance. However, she has no right in preventing her son from inheriting his father, be it from the house or other property.
It is an obligation on everyone to fear Allaah and abide by His Law, and divide the inheritance according to the Islamic Law. Nonetheless, an heir may renounce his share of inheritance otherwise the inheritance should be divided according to the Book of Allaah and the Sunnah of the Prophet .
We advise the heirs to take the issue to an Islamic court or to the nearest Islamic centre in order to study the case and divide the inheritance. We also advise you, dear questioner, to support your husband in being kind and dutiful to his mother and fulfil her rights on him.
Allaah Knows best.