Combine first 4 ingredients in a bowl; stir well with a whisk. Set aside.
Heat milk over medium-high heat in a medium, heavy saucepan to 180° or until tiny bubbles form around edge (do not boil). Remove from heat. Gradually add hot milk to sugar mixture, stirring constantly with a whisk. Return milk mixture to pan, and cook over medium-low heat until thick (about 8 minutes), stirring constantly. Remove from heat. Place pan in a large ice-filled bowl for 25 minutes or until egg mixture comes to room temperature, stirring occasionally. Stir in sour cream and vanilla.
Combine orange juice and amaretto. Split the ladyfingers in half lengthwise. Arrange 10 ladyfinger halves, flat sides up, in a single layer in the bottom of a 2-quart soufflé dish. Brush 2 tablespoons orange juice mixture over ladyfingers in the dish. Spread about 1 cup pie filling evenly over ladyfingers. Spread about 1 cup custard mixture over pie filling. Brush 10 ladyfinger halves with 2 tablespoons orange juice mixture, and line dish with ladyfinger halves standing upright. Arrange 10 ladyfinger halves over custard mixture, and brush with the remaining orange juice mixture. Spread remaining pie filling over ladyfingers. Spread remaining custard mixture over pie filling. Cover and chill for at least 8 hours.