Freddy’s son, Fred III, spoke at the funeral, and that night, his wife went into labor with their son, who developed seizures that led to cerebral palsy. The Trump family promised that it would take care of the medical bills.

Then came the unveiling of Fred Sr.’s will, which Donald had helped draft. It divided the bulk of the inheritance, at least $20 million, among his children and their descendants, “other than my son Fred C. Trump Jr.”

Freddy’s children sued, claiming that an earlier version of the will had entitled them to their father’s share of the estate, but that Donald and his siblings had used “undue influence” over their grandfather, who had dementia, to cut them out.

A week later, Mr. Trump retaliated by withdrawing the medical benefits critical to his nephew’s infant child.

“I was angry because they sued,” he explained during last week’s interview. At the time, he attributed their exclusion from the will to his father’s “tremendous dislike” for Freddy’s ex-wife, Linda. She and Fred III declined to comment on the dispute.