A Sydney woman has been ordered to stand trial after pleading not guilty to the stabbing murders of her father and sister.

The 26-year-old will also defend a charge of inflicting grievous bodily harm with intent to murder her mother.

The attack allegedly happened at the family's Revesby home in Sydney's south-west in July last year.

The Supreme Court was told mental health will be an issue at the trial, which will start in July.

It is alleged that the woman was denied psychiatric help because of her parents' belief in Scientology.