Must the grandfather's heirs be given a share of the land after it becomes clear that he relinquished it to his deceased son's wife and daughters?

It appears that the gift from your husband's father and his relinquishment of his share of his son's inheritance did not materialize, because it was not taken possession of during his lifetime, and the gift was not made known until after his death. A gift is not complete until possession is taken. If something is gifted to someone who has possession of it, and the donee is not informed until the donor dies, the gift becomes void.

This is unless the possession was already taken by the grandfather himself, as he is considered to be in the position of a father or guardian. If the grandfather took possession of what he gifted and had witnesses to it, then the gift is valid even if it was not made known until after his death, due to the completion of possession by him as a grandfather or guardian. His heirs would then have no right to claim it.

However, if he did not take possession of it himself, then the gift was not complete. In this case, you must give his share to his heirs, unless they willingly relinquish it and are mature and adult.