Complex sentence: Charles Leupp's brother-in-law, David W. Lee, who was also a partner in Leupp and Gould, took armed control of the tannery, based on his belief that Gould had cheated the Leupp and Lee families in the collapse of the business.
Split into simpler sentences: Charles Leupp's brother-in-law, David W. Lee, who was also a partner in Leupp and Gould, took armed control of the tannery. He believed that Gould had cheated the Leupp and Lee families in the collapse of the business.