At about 1530 CDT, the aircraft departed Mattoon, IL on a flight to Mankato, MN. However, the pilot landed at Albert Lea, MN where he refueled and topped the tanks with 45.5 gal of fuel. The aircraft's usable supply was 48 gal. While refueling, he reportedly told the airport manager that he thought he was at Mankato when he was landing at Albert Lea. Albert Lea was about 42 mi from Mankato. The airport manager stated that the pilot looked tired and that he (the manager) tried to persuade the pilot to continue the trip by automobile. The aircraft departed Albert Lea at about 2050 CDT. A short time later, the pilot contacted Mankato UNICOM and was provided landing information. During a night landing, the aircraft struck a power line about 65 ft AGL and 4327 ft from the runway, then nosed over and impacted in an inverted attitude. The two front seat occupants were fatally injured. Two young children in the back seats, both in child restraint seats, survived, one with minor injuries. RWY VASI LGTS on.