Is it obligatory to fulfill a father's will not to bury his son's wife in his grave, and would not fulfilling it be considered an act of disobedience?

It is not permissible to bury a person in a grave that still contains some remains of the first deceased, except in cases of necessity. It is also not permissible to exhume a grave to bury another deceased person until the first deceased has completely decomposed and their traces have vanished.

The grave is reserved for the one buried in it; therefore, one should not walk on it or exhume it as long as there is a trace of the deceased.

If the cemetery is private (family-owned), it is forbidden to bury someone in it without the permission of its owners, unless the living owners of the cemetery see a benefit in doing so.

The will of the deceased is not obligatory to execute except concerning their wealth, such as if they bequeathed the spending of one-third (of their estate) on a charitable deed. As for other bequests, they are to be presented to the Sharia (Islamic law); what is permissible is executed, and what is forbidden is prohibited.