Complex sentence: At the age of 13, he killed another family cat when he perceived it to be favoring his younger sister, Allyn Lee Kemper (born 1951), over him, and kept pieces of it in his closet until his mother found them.
Split into simpler sentences: At the age of 13, he killed another family cat when he perceived it to be favoring his younger sister, Allyn Lee Kemper (born 1951), over him. He kept pieces of it in his closet until his mother found them.