Hayley Hartson, 44, and Victoria Hartson De-Vulgt, 34, were among 11 people grilled by police after a flurry of bets on a league match between Welsh clubs Port Talbot Town and Rhyl.
A probe was launched after favourites Port Talbot lost 5-0 to their already-relegated rivals, who had not won in 17 matches.
The underdogs had lost 4-0 to their Welsh Premier League rivals just two weeks before the shock result in 2016.
Also questioned were Hayley’s son James, 22, and Victoria’s husband Leigh De-Vulgt, 36.
Stablelad Conor Murphy put a £50 accumulator bet on five of his bosses’ horses to win at the Cheltenham Festival in 2011. He received a £1m payout, and used the money to become a top trainer himself
“We haven’t been charged with anything”
Both men played for Port Talbot but were not picked for the game.
Hayley, whose ex-Celtic, West Ham and Wales star brother expects to go to Gamblers Anonymous meetings until he is 70, said: “We haven’t been charged with anything. It is what it is. I haven’t been charged with anything and neither has my son or my sister.”
Football chiefs called in gambling experts and police to investigate the game after the upset surprised pundits.
Officials studied video footage of the match which shows “very sloppy” defending for all five goals.
Bookies were offering between 70-1 and 100-1 against Rhyl winning 5-0 in the match.
Large sums were placed on Rhyl to be leading at half-time and full-time at odds of 8-1.
South Wales Police said it had “submitted a file of evidence to the Crown Prosecution Service as part of an investigation into alleged match fixing of a Welsh Premier League fixture”.