A man has driven his car into his sister during a domestic dispute in New South Wales, sparked when her cat urinated on his computer, police say.

The siblings began fighting in their house on Heather Road, Winmalee, north west of Sydney, about 2.30pm (4.30pm NZ time) yesterday.

Police said the 19-year-old woman was dragged across the floor, before the man went outside and threw the cat into his car.

When the woman ran onto the street, her 20-year-old brother allegedly drove into her, police said.

She was taken to Nepean Hospital in a serious condition, with a fractured spine, fractured leg and a collapsed lung.

Blue Mountains local area command duty officer Inspector Peter Scheinflug said it was unclear what the man was planning to do with the cat when he threw it in the car.

"There's no such thing as a routine domestic. They're all unpredictable," Inspector Scheinflug said.

"It's an unusual one."

Police established a crime scene after the alleged fight yesterday and arrested the brother.

He was charged with common assault and negligent driving and was granted bail to appear in Penrith Local Court on May 15.

The cat was not hurt.

Inspector Scheinflug said the cat did not go to an animal shelter.

"The cat got out of the car and it's safe and well."

- Sydney Morning Herald