When Hainan Airlines wanted to expand in the 1990s, its top executives turned to a little-known businessman in his early 30s, who helped the Chinese upstart develop golf courses, resorts and seaside villas.

It was the beginning of a lucrative partnership for the businessman, Wang Wei, one that would put him on the path to becoming a billionaire. Over the next two decades, Mr. Wang set up companies that managed properties, as well as supplied computers, software services, seafood and Cuban cigars to Hainan Airlines and its parent company, the HNA Group.

What HNA rarely publicly revealed, if ever, is that Mr. Wang is the younger brother of its co-founder and co-chairman, Wang Jian. In the past 25 years, HNA has regularly funneled business to a small group of relatives and associates of the company’s senior executives, dealings with limited disclosure to investors in its listed companies or its overseas bonds, according to a review of thousands of corporate records by The New York Times.

From its roots as a small airline, HNA has evolved into one of the few Chinese conglomerates with a worldwide reach. The company, which produces $100 billion in annual revenue, has made multibillion-dollar deals in the United States, Europe and the rest of Asia, amassing big stakes in multinationals like Deutsche Bank and Hilton Hotels.