Is it permissible to write a will whereby the mother sells her house to build a mosque, which represents more than two-thirds of her inheritance, and is it permissible to conceal this will from the rest of the heirs?

There is no harm in writing the mother's will, especially if the mother requests it.

A will is not validated without attestation unless the heirs acknowledge it after the mother's death. Scholars differ on the number of witnesses required to validate a will. Some say it is validated by one man and two women, or one man and an oath, while others say it is only validated by two men.

A will is not valid for more than one-third of the inheritance, and only one-third of it can be executed, based on the hadith of Sa'd ibn Abi Waqqas with the Prophet, peace be upon him.

If the mother is in a life-threatening illness, she does not have the right to dispose of more than one-third of her estate. She may be legally restricted (interdicted) to protect the rights of the heirs.

If the mother is healthy, she has the right to dispose of her properties, and her intention to disinherit heirs does not prevent her from doing so.