Is “indifferent to negligent” just a set of word? How different is it from “indifferent and negligent”?

There was an article titled “The short life and lonely death of Sabrina Seelig” in July 29 New York Times reporting the death of a young woman who was a student of classics at Hunter College. She was carried to a hospital by ambulance after feeling sick, and died there. The article says:

“The family of Ms. Selig, 22, says she suffered an agonizing death because the care she received at a struggling Brooklyn hospital (Wyckoff Heights Medical Center) was indifferent to negligent.”

I thought the expression, “indifferent to negligent” an idiom. I checked NGram. It doesn’t register this phrase, neither “indifferent and negligent,” or “indifference and negligence” which were obviously used in the following ways:

• Could a husband be indifferent and negligent towards a wife during pregnancy...at a time when she has chosen to bear his offspring in her womb? Can he consider her a liability? －Redif com.

• President Ilves: volunteers stand like a protective wall against indifference and negligence. Press release from the Office of President of Republic of Estonia 7/12/2012

Is “indifferent to negligent” a simple repetition of synonyms for emphasizing the meaning? Then, how different is “indifferent to negligent” from “indifferent and negligent”?

The phrase "indifferent to negligent" is a phrase describing a range of possible adjectives for the given situation. That is, the quote might be rewritten:

the care that Ms. Selig received ranged from indifferent to negligent

The phrase itself isn't idiomatic, but the construction is. It isn't the same as "indifferent and negligent", which pinpoints the two adjectives to describe the care. The to presents a spectrum of ways to describe the care.