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The allusion is found in the second paragraph.
"I incline to Cain's heresy," he used to say quaintly: "I let my brother go to the devil in his own way." In this character, it was frequently his fortune to be the last reputable acquaintance and the last good influence in the lives of down-going men. And to such as these, so long as they came about his chambers, he never marked a shade of change in his demeanour.
This allusion directly relates to the storyline in that foreshadows Jekyll's (the good brother) need to murder Hyde (the evil brother. In the Bible, however, the evil brother murders the good brother.
Dr. Jekyll and Mr. Hyde