Les parents n'avaient cessé de râler et de se disputer

Jennifer, 

" cesser ' is one of the very few verbs that can be routinely expressed in the negative with just the use of the ' ne littéraire ' - that is without needing " pas " for the negation. It is used in this manner in formal writing, but not usually in speech, especially not everyday speech. 

Just the sort of usage you find at C1 level ! 

See link for further discussion by Laura Lawless:

https://www.lawlessfrench.com/grammar/ne-litteraire/