The Bidens are hardly alone. President Trump’s transportation secretary, Elaine Chao, and her husband, Senator Mitch McConnell, are being accused of having profited from their commercial ties to Beijing. In 2004, the two had a net worth of about $3.1 million, according to public disclosures. Three years later, the range was $3.1 million to $12.7 million. The next year, their net worth rocketed to $7.3 million to $33.1 million.

What changed? In 2008, Ms. Chao’s father, James Chao, gave the couple a “gift” of $5 million to $25 million (politicians are required to report money in ranges, not exact amounts). Certainly, their wealth has continued to grow.

Mr. Chao’s generosity was made possible by the fortune he has amassed through his shipping company, Foremost Group, which has thrived in large part because of its close ties with the Chinese government. In late 1993, Mr. Chao and his son-in-law, Mr. McConnell, traveled to China as guests of the state-owned shipyard conglomerate and military contractor, China State Shipbuilding Corporation. There they met with an old classmate of Mr. Chao’s, the former Chinese president Jiang Zemin.

Mr. McConnell’s once hard-line condemnations of China softened in the years to follow. For example, as The New Republic has noted, Mr. McConnell went from telling University of Louisville students that America would never forget Tiananmen Square, in the late 1980s, to hosting the Chinese ambassador at the same school several years later, even as the ambassador publicly defended the regime’s suppression of the Falun Gong.

All along, the Chaos continued to gain influence. Mr. McConnell’s sister-in-law, Angela Chao, and James Chao sat on the board of the holding company for China State Shipbuilding. While Elaine Chao was secretary of labor under George W. Bush, Foremost Group ordered several enormous cargo ships from a subsidiary of China State Shipbuilding. Secretary Chao and her father also appeared in several tandem interviews with Chinese media, and in at least one, they sit in front of the Department of Transportation’s emblem and alongside images of a book written by Mr. Chao. Today, Angela Chao sits on the board of the Chinese government’s Bank of China.

Last month, the House Oversight and Reform Committee started an investigation into whether Secretary Chao has leveraged her government positions to benefit her family. But so far there is no investigation into Joe and Hunter Biden. Defenders claim there must first be proof that a law was broken to open an investigation. That’s exactly backward. Congress can and should conduct an inquiry to determine whether anything illegal occurred.

The problem more broadly is that we rely on a hodgepodge of laws that lack the clarity and bright ethical lines found in the Foreign Corrupt Practices Act. That needs to change. International bribery laws clearly state that if a person or entity pays a politician’s family member and gets favors in return, it’s an act of bribery; it’s no different from the politician taking the money himself.