While the two were young, their father took them out on a camping trip, which unfortunately led to the death of their father. Angela and Peter managed to survive.
Peter would get adopted by his Aunt Martha, who sought to raise him as the girl she had never had, and taking his sister's place.
Peter was mentally unhinged, and became widely known as "Angela Baker", and began killing sprees.
The real Angela Baker was the killer in Return to Sleepaway Camp.