A man has driven his car into his sister during a domestic dispute in the Blue Mountains, 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 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.