Is it permissible for a mother to claim her right to her father's inheritance after she relinquished it to her brothers 25 years ago and signed the waiver agreement 5 years ago?

If a mother relinquishes her share of her father's inheritance after his death, having already taken ownership of it, and this relinquishment is voluntary and without coercion, then this is a valid gift (hiba) to her siblings as soon as they take possession of it. It is not permissible to revoke it, based on the Prophet's (peace be upon him) saying: "Do not go back on your charity," and "One who retracts his gift is like a dog that returns to its vomit." However, if the relinquishment occurred before the father's death, or if she retracted before her siblings took possession of their share, then she is permitted to retract, because a gift is not binding until it is seized and possessed.