Gazprombank declined to comment, and Gazfond did not respond to questions.

Timchenko Connection

On top of their family links, the brothers Shamalov have deep connections to other members of the Putin elite who reshaped Russia's economy in the 2000s and made personal fortunes doing so. One is Gennady Timchenko, who became a shareholder in Bank Rossiya along with Nikolai Shamalov.

Timchenko has known Putin for more than 20 years. In the 1990s, he began oil trading from St. Petersburg, when Putin was a rising politician there, and went on to co-found Gunvor, a company that grew to be one of the largest traders of Russian oil. Last year, the U.S. government alleged that Putin had a personal stake in Gunvor, though it offered no evidence of this. Gunvor denied the allegation.

Timchenko was an important contact for Kirill Shamalov, because the oil-trading magnate later became a large shareholder in Sibur.

In 2010, Gazprombank and Yuri Shamalov's Gazfond held a combined 95 percent stake in Sibur. That year, they made a deal to sell Sibur to a company owned by Timchenko and one of his business partners, an energy entrepreneur named Leonid Mikhelson. The deal was complex, but Gazprombank smoothed the way by lending money to Mikhelson and Timchenko to fund at least half the purchase.

Mikhelson ended up owning 57.2 percent of Sibur and Timchenko 37.3 percent. Five managers and former managers owned the remaining 5.5 percent. Those managers included Kirill Shamalov. By 2014, according to Sibur's declarations, Kirill had extended his personal stake to 4.3 percent of Sibur.

Kirill's Coup

Once Mikhelson and Timchenko had taken control of Sibur, the only way for anyone to obtain a major stake in the company was for one of them to agree to sell some shares. Fortunately for Kirill Shamalov, Putin's friend Timchenko was willing to sell to him.

Timchenko and the president's new son-in-law began discussions in the summer of 2013, according to an interview Kirill gave to the Russian newspaper Kommersant in August this year. The start of those talks was a few months after Kirill had celebrated his marriage to Putin's daughter.

In March 2014, Kirill stepped down from his management role at Sibur. But he kept his shareholding and stayed on as member of the board of directors. Four months later, on Aug. 1, he registered a new company called Yauza 12 that is wholly owned by him.

The following month, Yauza 12 bought 17 percent of Sibur from Timchenko. That took Kirill's stake in the petrochemical company to 21.3 percent. Putin's son-in-law was now the second largest shareholder in Russia's leading gas and petrochemicals processor.

Earlier this year, a spokesman for Timchenko told Reuters that his sale to Kirill Shamalov took place after negotiations with several potential buyers and was at a market price. The spokesman said the sale reflected a policy of Timchenko's holding company, Volga Group, of diversifying its assets.

This month, a spokesman for Volga Group told Reuters: "All transactions which were conducted by Mr Timchenko's structures with shares of Sibur - the acquisition of shares in the company, the sale of part of the share structure to K. Shamalov - had a market character and took place based on the market valuation of the company."

How could the then-32-year-old Kirill afford such a purchase? A Reuters examination of Yauza 12 accounts filed at the end of 2014 shows that the company had borrowed 78.9 billion roubles (about $1.3 billion at the time). The source of this money was the bank where his brother is a deputy chairman. The financing came "from Gazprombank secured on assets belonging to me," Kirill told Kommersant.

Yury Shamalov did not respond to requests for comment.

The terms of the loan are not known, but Kirill's company appears to have borrowed the money cheaply. Interest paid after he took the loan, according to the Yauza 12 accounts, amounted to 343 million roubles to the end of 2014. That equates to an annual interest rate of about 1.3 percent if the loan began when Yauza 12 bought the Sibur shares, a Moscow-based credit analyst said. The analyst added that because the loan's maturity date is unknown, it is unclear whether that interest rate differs from the market rate or not.

Kirill Shamalov and Gazprombank declined to respond to Reuters questions about the loan.

Using the borrowed $1.3 billion, Kirill was able to buy the 17 percent stake in Sibur. He and his spokesman declined to specify how much he paid, other than saying it was a market price.

Three independent analysts told Reuters in October this year that Kirill's total holding of 21.3 percent was worth at least $2 billion. That may have been conservative. On Dec. 11, the Chinese petroleum company Sinopec agreed to pay $1.34 billion for only 10 percent of the company.

That valuation implies that Kirill's overall 21.3 percent holding in Sibur is now worth $2.85 billion.

Full Circle

With his secret wedding at the Igora ski resort and his financial good fortune, Kirill Shamalov encapsulates key aspects of today's Russia. The Putin era has its roots in St. Petersburg, where the president began his political career. His friends from the Ozero dacha cooperative, as well as aides he worked with in the St. Petersburg mayor's office, went on to wealth and influence as Putin consolidated his power.

Now Kirill has joined the ranks of the billionaire elite around the president. The connections, though, are kept discreet. When Kirill and Katerina Tikhonova celebrated their wedding, security was tight and the guest list exclusive: About 100 attended, according to people who were there.