An inquest in Belfast heard that the incident began when Graham Spence entered the tank to find a collie dog that had fallen in.
Emma Rice, who was also overcome by the poisonous gases when she climbed down a ladder to try to find her father and brothers, told Northern Ireland's Senior Coroner John Leckey that she knew how dangerous it was to go into the pit.
"When it comes to the love of your family, it doesn't matter," she said.
After helping to pull her father from the pit, Mrs Rice passed out and fell back into the slurry as she tried to bring Graham to safety.
She was rescued by neighbours who had rushed to the scene to help.
Mr Leckey said Mrs Rice's actions were "extremely brave".
The inquest was told that Nevin and his brother had been loading wood into the Ulster star's car, helped by friend Andrew Oliver.
Mr Oliver and Nevin then went into the farmhouse after the rugby player's mother called him for his dinner.
Noel Spence then came into the house and said the dog had fallen in the tank.
Northern Ireland's State pathologist Professor Jack Crane and investigators from the region's Health and Safety Executive are scheduled to give evidence when the inquest concludes tomorrow.