The 1952 law under which the Trump administration seeks to deport Mahmoud Khalil, a lawful permanent resident who helped organize protests at Columbia University, is largely untested.

Largely, but not entirely. It was ruled unconstitutional in 1996 — by President Trump’s sister.

Mr. Trump does not have much use for a lot of judges. Last week, for instance, he called for the impeachment of “many of the Crooked Judges I am forced to appear before.” But he held his sister, Judge Maryanne Trump Barry, in high regard.

“I will never forget the many times people would come up to me and say, ‘Your sister was the smartest person on the Court,’” he posted on social media when she died in 2023. “I was always honored by that, but understood exactly what they meant — They were right! She was a great Judge, and a great sister.”

When Judge Barry considered the 1952 law, which the Trump administration has said will play a major role in its deportation plans, she asked whether it could be squared with the Constitution. “The answer,” she wrote, “is a ringing ‘no.’”

At the time, Judge Barry was a federal trial judge, and so her ruling did not establish a precedent binding on other courts. In any event, an appeals court later reversed her decision, though on grounds unrelated to its substance.

But it remains the most thorough judicial examination of the constitutionality of the law, and other judges may find its reasoning persuasive.