Complex sentence: In 1978 the Pointer Sisters had begun an eleven year association with RCA-first with producer Richard Perry's Planet label and then still working with Perry with the parent company-with the group amassing a total of eleven Top 40 hits.
Split into simpler sentences: In 1978 the Pointer Sisters had begun an eleven year association with Richard Perry. First through hisPlanet label and then still working with Perry with RCA-with the group amassing a total of eleven Top 40 hits.