Is it obligatory to carry out the deceased's will not to bury his sister in his private burial plot, or is this a will that involves injustice and thus must not be carried out, especially given that the sister has other burial plots available to her?

The land becomes an endowment (waqf) if a man designates it as a cemetery for himself and his family. An endowment is established by a declaration or an action indicating it, such as saying: "This cemetery is an endowment," or by permitting one of his family members to be buried in it. Once the endowment is established, the land ceases to be owned by the endower and his heirs, and they are not permitted to alter the conditions of the endowment, as it is a binding contract. The conditions of the endower must be adhered to regarding who is buried in the cemetery, unless they contradict Islamic law. Therefore, as long as the grandfather endowed the land to be a cemetery for himself and his family, no one from outside his family may be buried in it, and no family member may be deprived of the right to be buried there.