Call them the sad siblings. Senator Ted Cruz’s half-sister Miriam died of an overdose in 2011 after a struggle with drug addiction and a series of arrests. Governor John Kasich’s brother Richard has schizoaffective disorder and receives government disability benefits.

Hillary Clinton’s brother Tony Rodham “had not made a mortgage payment in 10 months and was fighting home foreclosure,” the New York Times reported.

Donald Trump’s older brother Freddy “died as an alcoholic in 1981 at the age of 43,” the Times reported. (Even Donald Trump’s wife Melania Trump has a half-brother, Denis Cigelnjak, living in what GQ magazine’s intrepid Julia Ioffee, who discovered him, describes as a “tiny apartment” in Hrastnik, Slovenia.)

The sorrows of the siblings are hardly unprecedented. President Bill Clinton pardoned his half-brother Roger, who had been convicted of a cocaine-related offense. President Carter’s brother Billy publicly struggled with a drinking problem, became entangled in a scandal over his paid dealings with the government of Libya, and died of pancreatic cancer at age 51. President Obama’s half-brother George was a gangster in a Nairobi slum.

But as a matter of public policy, the struggles of the political siblings are worth some attention in this campaign season for the perspective they offer on the challenges our country faces.

The problems faced by these siblings call into question one of the more facile narratives about poverty and disease in America, which is that such hard-luck stories are explained primarily by childhood circumstances. If only hard-luck-story-exhibit A had been born into a better family, home, or neighborhood, and had better educational opportunities, the story goes, he’d be more successful, and wouldn’t be on welfare, or in rehab, or in jail, or unemployed on some street corner.

Well, these examples show, it isn’t so. Freddy Trump had the same parents that Donald did, but Freddy ended up a dead alcoholic, while Donald ended up as a wealthy presidential candidate. Tony Rodham and Hillary Rodham Clinton had the same parents, but Tony hasn’t had much success, while Hillary is a wealthy former senator and secretary of state.

One can’t blame racism or the legacy of slavery for the misfortunes of Richard Kasich or Miriam Cruz. One can’t blame an “underclass” isolated from mainstream middle-class American culture.

Journalists, particularly at the Times, of which I am often critical, deserve some credit for delving into the human details of these stories. The accounts share some recurring themes. Family members with ample resources did their best to help out — paying for rehabilitation or for school for children, or even, in Senator Cruz’s case, showing up in person at a run-down motel to stage an intervention. Yet even that isn’t always enough.

Why do some siblings succeed and others fail? The question goes back at least to the biblical Cain and Abel, along with the lack of satisfactory answers. If not even loving and in some cases rich, famous, or politically powerful parents and family members can assure a positive outcome for their own children or kin, what hope is there that government, or community organizations, will achieve better outcomes?

Where the problem is substance abuse, there seems to be an emerging political consensus that increased availability of treatment, and funding for it, is desirable. Same for mental health. One way to think about those issues is that they are like cancer, or heart disease. Research is improving health outcomes, but perfect cures remain elusive, despite the deployment of immense resources in the form of government, non-profit, and corporate research and treatment funds.

There’s a certain hubris to politicians promising to help with national problems of poverty, substance abuse, and mental illness that they haven’t been able to conquer fully even in their own families. But even gradual progress on these fronts will require both the humility and the sense of urgency that is acquired, painfully, by dealing with these problems at close range. In that limited but nonetheless important sense, at least, the current crop of candidates may yet turn out to be a highly qualified group for which Americans can be thankful.

Mr. Stoll is editor of FutureOfCapitalism.com.