Complex sentence: Oscar Homolka was originally cast as Holmes's elder brother Mycroft, but for family reasons he quit the part and Mycroft's character was replaced by "Mr. Ahrens" played by Holmes Herbert.
Split into simpler sentences: Oscar Homolka was originally cast in this film as Holmes's elder brother Mycroft. He quit the part due to family issues, and Mycroft's character was replaced by "Mr. Ahrens", portrayed by Holmes Herbert.