The answer is to help the reader visualize a house bordered by smaller cottages
He was spending his summer vacation, as he always did, with his mother at Grand Isle. In former times, before Robert could remember, "the house" had been a summer luxury of the Lebruns. Now, flanked by its dozen or more cottages, which were always filled with exclusive visitors from the "Quartier Français," it enabled Madame Lebrun to maintain the easy and comfortable existence which appeared to be her birthright.
The author uses the word flanked meaning, according to Cambridge "to be at the side of someone or something" to let the reader imagine a large, luxurious and expensive house surrounded by small cottages but equally important since they were exclusive to the inhabitants of the French quarter.
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