We know about Hegseth's brother and wife due to people in the Pentagon leaking these stories to the press
https://www.rawstory.com/loose-lipped-pentagon-staff-hit-hegseth-with-a-crisis-equal-to-signalgate-analyst
Some Pentagon officials seem willing and able to leak sensitive information about Defense Secretary Pete Hegseth's shortcomings and failures and it's creating scandals just as hefty as Signalgate, according to a new article with MSNBC.....
Also last week, The Wall Street Journal published an unflattering report that Hegseth "brought his wife, a former Fox News producer, to two meetings with foreign military counterparts where sensitive information was discussed," the Journal wrote, citing "multiple people who were present or had knowledge of the discussions."
Politico and CNN published their own stories saying that, while President Donald Trump outwardly expressed support for Hegseth, interviews with administration insiders "reflect growing concerns about Hegseths leadership at the Pentagon.
Benen wrote that all of these stories pointed to the fact "that there are some people at the Pentagon whove learned some unfortunate things about Hegseth, and theyre suddenly eager to share that information with journalists."
He concluded, "Its a problem for the Pentagon chief that theres evidence of his incompetence and failures, but its just as notable a problem that there are DOD insiders who are apparently willing to talk to reporters about his incompetence and failures."