"We don't want others to go through this ... tell someone, talk about it and get it out there," Mr McGrath told the Northern Advocate.
"It's terrifying that this could happen."
Those gathering at Ms McGrath's tangi, scheduled for Saturday at Korokota Marae at Titoki, will be given a white ribbon - the symbol of a global campaign against violence towards women.
"People who suspect domestic violence should raise the alarm and get out so they aren't in the same position as my sister," Mr McGrath said.
Phillip Andre Mahanga, 32, has been charged with assaulting Ms McGrath and been remanded in custody until January 21, when more serious charges are likely to be laid by police.
The family said Ms McGrath and Mahanga had been in an "on again, off again" relationship for 14 months.
Mr McGrath said there was no malice towards Mahanga's family as he believed they were confused and hurting as well.
"We have pleaded for members of our family to be strong and let the proper channels deal with the matter."
Her two children Brooke, 15, and Payton, 12, are with their father, Gareth Barton, who was her partner for 16 years.
Ms McGrath was described as a "people's person" and her desire to help others led her to becoming a case manager with Work and Income in Whangarei for five years.
Last year she had travelled to Christchurch following the February 22 earthquake to help her counterparts there deal with the aftermath.
"She was a strong person and she made sure people were looked after. She probably overfed people but that's just her mother rubbing off," Mr McGrath said.
She was community minded and had previously chaired her local marae committee.
Her brother Albert Tuhiwai said his sister "was probably the most beautiful girl in our life".
"She was definitely out there to help other people better their lives," Mr Tuhiwai said.
Jane Tuhiwai described her daughter as determined and always keen to debate.
"She's a beautiful girl and a real people's person. It's not what she said, but what she did."
It is not the first time the family have dealt with tragedy. In 1979, Robert McGrath and 9-year-old daughter Patricia Ann were killed in a car crash in Auckland. Mrs Tuhiwai called her daughter Patricia Ann in honour of the older sister she never met.
John McGrath survived the crash and said Patricia Ann had been their second chance at having another sister.
"We are grateful we had her for 34 years," said Mr McGrath.
White Ribbon: It's OK to ask for help - call 0800 456 450
