What is the MAIN reason WHY the author
includes the section "Two sets of sisters"?

That would be plagiarism

Prewriting: this stage of the writing process involve gathering and selecting ideas; teachers can students in several ways: creating lists, researching, brainstorming,reading to discover more about the author's style, talking, collecting memorabilia or clips from other texts, and free-writingdrafting: in this stage, students begin writing, connecting, and developing ideasrevising: this is the stage of writing that involves rewriting or "re-seeing; " emphasis is place on examining sentence structure, word choice, voice, and organization of the pieceediting: this stage involves checking for style and conventions--spelling, grammar, usage, and punctuationpublishing: the "going public" stage of writingevaluating: in this stage, the writer looks back at his/her work and self-evaluates, and the audience evaluates the effectiveness of the writingwriting activities : personal writing, workplace writing, subject writing, creative writing, persuasive writing, and scholarly writingpeer review: acting a referee; evaluating a colleague's work professionally