Of the two brothers, Anil is the more flamboyant and outgoing, while Mukesh is regarded as more staid  less likely, at least, to build at 27-story house for himself. The new tower is located on Altamount Road, the same leafy residential street in south Mumbai where the father bought his first home after moving the family out of the tenements. Later, he purchased a 14-story apartment building named Sea Wind, where both Mukesh and Anil have lived with their families on different floors, even during their feud. (Their mother refereed from her own residence in the building.)

Now Mukesh is moving into a tower that makes Sea Wind seem like a guest house.

“It’s kind of returning with a vengeance to where they made it into the middle class and trumping everybody,” said Hamish McDonald, who chronicled the family’s history in his new book, “Mahabharata in Polyester: The Making of the World’s Richest Brothers and Their Feud.”

“He’s sort of saying, ‘I’m rich and I don’t care what you think,’ ” Mr. McDonald said.

Mumbai, once known as Bombay, is India’s most cosmopolitan city, with a metropolitan area of roughly 20 million people. Migrants have poured into the city during the past decade, drawn by Mumbai’s reputation as India’s “city of dreams,” where anyone can become rich. But it is also a city infamous for its poor: a recent study found that roughly 62 percent of the population lived in slums, including one of Asia’s biggest, Dharavi, which houses more than one million people.

Real estate prices are among the highest in the world, pushing many working-class residents into slums, even as developers have brazenly cleared land for a new generation of high-rise apartment towers for the affluent. High-rises are considered necessary, given the city’s limited land, yet the rising towers have further insulated the rich from the teeming metropolis below. With his helipads, which still await operating approval, Mr. Ambani could conceivably live in Mumbai without ever touching the ground.

“This is a gated community in the sky,” said Gyan Prakash, author of the new book “Mumbai Fables.” “It is in a way reflective of how the rich are turning their faces away from the city.”

Along Altamount Road, which is also home to other industrialists, the reaction to the new neighbor is mixed. Some senior citizens along the street worry about the noise from the comings and goings of helicopters. But Utsav Unadkat and Harsh Daga, college students who grew up in the neighborhood, stared up at the tower on a recent afternoon as if it were a dream realized.