You have identified an incredibly important aspect of the way in which this unforgettable novel is written. What is distinct about the narrative style of Roy in this book is that she abandons a chronological narrative in favour of extensive use of flashbacks and digressions as the story shifts from the present to the past. The way in which these separate elements are woven together to tell the story of the Kochamma family and unfold the tragedy that Estha and Rahel are still living under is a mark of the excellence of this novel.
The first narrative time frame begins when Estha and Rahel are children and their cousin, Sophie Mol, is just about to arrive. The second beings after Estha has been "re-returned" by his father after a gap of twenty three years. Note what the novel says about this gap:
Two weeks later, Estha was Returned. Ammu was made to send him back to their father, who had by then resigned his lonely tea estate job in Assam and moved to Calcutta to work for a company that made carbon black. He had remarried, stopped drinking (more or less), and suffered only occasional relapses.
Estha and Rahel hadn't seen each other since.
It is this gap of 23 years that Roy crosses at will with her narrative, uniting the present with the past and showing how the childhood experiences of Estha and Rahel still bear a massive mark on their lives in the future.