Can someone come back for anything after they sign a letter excepting what they got?

My father was made executor of the estate and he wrote a letter to his one sister and it says. “Estate Of, I, Acknowledge receipt of $ from Executor of the estate of” I accept this amount in full payment and satisfaction of my interest in the estate. His sister signed it and it was notarized. Now months later she filed an Evocation of Wavier of Notice and demand for Inventory and Accounting letter wanting an inventory and accounting of the assets of the estate in addition to a copy of all title company closing documents for the sale of the estate property. Can she come after my father for anything else?