Rachel's father said doctors had ruled out a genetic factor in Rachel's death and the results for the twins' tests had come back clear.
Doctors were trying to establish if her death was caused by a virus.
"If it was a genetic thing we would obviously be concerned for our other daughters but the doctors seem to have ruled that out," Tony Clark said told the Herald on Sunday.
Microscopic tissue samples from Rachel have been sent to laboratories here and in Australia to try to find out why the fit and healthy teen died suddenly.
Rachel's father said post mortem examination reports showed Rachel had mild cardiomegaly dilatation - an inflammation of tissue around the heart.
"That's why they sent tissue away, but we don't know if they will establish a cause of death at all," Clark said.
"We are resigned to the fact we may never find out why Rachel died."
More than 800 people attended Rachel's funeral, with Grammy Award-winning popstar Lorde so touched she sent flowers for her coffin.
On Tuesday the heartbroken family gathered with a group of Rachel's friends to celebrate what would have been her 18th birthday.
"We got a group of us together and climbed one of the local hills and watched the sun come up," Clark said.
He said a huge amount of community support since Rachel's death had helped the family cope. "Everyone has been absolutely amazing and Rachel's friends have been over to visit a lot," Clark said.
A Rachel Clark Memorial Trust had raised $11,000 which would go to a deserving cause at the talented rower's school.
Donations can be made to the Rachel Clark Memorial Trust at ANZ 060 92101 15451 00.
