A flashing system including a pre-molded corner flashing member made from polymer material in a single continuous seamless piece. The flashing member has interior flashing portions that meet at an interior corner to conform with a corner of the rough opening, and a membrane flashing portion intersecting each of the interior flashing portions along separate exterior corners, the interior and exterior corners intersecting at a common point. The interior corner of the opening may be sealed by placing the molded corner flashing member in the corner and then running a strip of preferably pre-creased sealing tape or other sealing member along the periphery of the wall opening and overlapping a portion of the sealing member on both the interior flashing portion and the membrane flashing portion.