By Shantanu Guha Ray, Delhi
The brothers have been called the 'squabbling siblings'
They are the world's richest brothers with a combined wealth of $85bn - and they are fighting again.
Two years ago, Mukesh Ambani and Anil Ambani, who control India's Reliance group of industries, accused each other of lying, misleading shareholders and delaying the process of dividing the businesses between them.
The latest rivalry between chief of Reliance Communications Anil Ambani and his elder brother Mukesh could upset a $70bn (£36bn;45bn euros) merger between Reliance Communications and the South African mobile telephone company, MTN.
There have been suggestions that Anil Ambani would sell a stake in Reliance Communications to MTN.
But Mukesh Ambani, India's richest man, says he has the first right to buy a stake in his younger brother's firm.
He says this is laid down in a family agreement. Anil Ambani says the agreement was never ratified.
India's corporate observers say central to the fresh round of rivalry is the issue of wealth and one-upmanship between the two brothers.
In the Forbes list of billionaires, Mukesh Ambani holds the sixth position, followed by Anil Ambani.
Anil Ambani's personal wealth stands at $42bn, just $1bn short of his elder brother's. And if the MTN deal goes through, he may well leave his brother way behind.
When the Reliance empire was divided in 2005 after a bitter seven-month feud, Mukesh Ambani, who built the world's largest refinery at Jamnagar in India's western Gujarat state, was described in the media as the hard-working son whose worth was thrice that of his flashy younger brother's.
Newsweek magazine quoted his employees calling Mukesh, The Boss, who reportedly bought his wife a Boeing jet as birthday present and is building a $1bn, 27-storey home in Mumbai that many claim is the world's most expensive house.
Younger brother Anil Ambani has not lagged behind.
Mrs Ambani has mediated in resolving disputes between her sons in the past
A vegetarian and teetotaller, his Reliance Communications is one of India's top telecom companies.
The junior Ambani has expanded his energy interests through Reliance Energy that distributes power in parts of the Indian capital, Delhi, and Mumbai.
And his media company, Reliance Big Entertainment, has announced production deals with Tom Hanks, Brad Pitt, George Clooney and Nicolas Cage. One recent report said Anil Ambani was putting his money in Steven Spielberg's Dreamworks.
Anil Ambani has recently signed a mega deal to fund new ventures with some of India's biggest actors, Amitabh Bachchan, his son Abhishek and daughter-in-law Aishwarya Rai.
The two brothers inherited their multi-billion dollar conglomerates from their father, Dhirubhai Ambani, who died in 2002.
Since the late Mr Ambani left no will, the business was divided by their mother, Mrs Kokilaben Ambani, after it became obvious the two brothers would not be able to run the sprawling company together.
So will Mrs Ambani step in again to resolve the latest dispute between her sons?
Earlier this month, she helped her younger son host a dinner at Christie's and sat next to MTN chairman Phuthuma Nhleko - widely interpreted as her blessing for the deal.
Observers say she visited family friends in the US and skipped the elder son-controlled Reliance Industries' annual general meeting.
Time will tell whether she will intervene again to solve the latest row between what the Indian media describes as the "squabbling siblings".
The writer is business editor of Tehelka magazine.