What is the religious ruling concerning the debt that my deceased sister owed me, the amount of which is unknown to me and was not recorded, and she left behind a daughter, a mother, and siblings who are claiming their inheritance; does the debt become nullified or must it be repaid?

If you do not have proof of the debt, then your sister's heirs are not obligated to pay it. However, if one is strongly convinced of the debt's existence, then religiously, they are obliged to pay their share of it. If the heirs acknowledge the debt but its amount cannot be ascertained, it is best to reconcile on an amount that seeks to ascertain the truth, because reconciliation over an unknown amount is permissible according to some jurists, including Ibn Qudamah, if its knowledge is impossible.