"Is that how a groom should be celebrating the best day of his life?" asked his distraught sister, Christine Sarkis, in a victim impact statement read out in the NSW Supreme Court on Friday.

Her cousin, Katherine Abdallah, was facing a sentence hearing after being found guilty at her retrial in December of the manslaughter of 21-year-old Suzie Sarkis in February 2013.

Abdallah had been involved in a violent and prolonged argument over her new $184,000 Mercedes before she armed herself with two knives and killed her cousin in Abdallah's Sydney townhouse.

The jury rejected the now 36-year-old's claim that she acted in self-defence.

In her victim impact statement, Christine Sarkis said her baby sister was killed by someone who used to tell them that "you girls are my family, I love you".

"I feel betrayed," she said.

"I wish she had died in an accident and not killed by someone who I was so close to."

Her brother got married last year but their sister was missing and he "had to go to his sister's grave for her to be included in his special day," Ms Sarkis said.

She lamented never again being able to fight with, kiss, hug or annoy her "like only a big sister could".

"I don't understand how a person could do this to her family," she said.

"Is that person full of jealousy, is that person full of hatred or are you simply evil?"

A statement was read out on behalf of Suzie's mother, Mary Sarkis, who wrote of her nightmares and of pretending her daughter was still alive.

Her room was left exactly the way it was before she died, but the mother said she had only been in it three times when she could smell her daughter's perfume.

"I thought Suzie would be safe where she was staying (with Abdallah) but someone in the family stabbed her and caused her death," she said.

Abdallah will be sentenced by Justice Julia Lonergan on a later date.

Australian Associated Press