Both sisters told their parents of the catalogue of abuse they suffered, but they did nothing to prevent future assaults.
The mother said she could not step in because one of the men “helped them with money and a car”.
A jury at Manchester Minshull Street Crown Court found the couple failed to protect the girls.
The adoptive father was also convicted of sexually abusing the elder girl over a six-year period.
Last year, Colin Molloy and Sam Nelson were jailed after pleading guilty to a string of sexual offences committed against the two youngsters, which started when they were as young as seven and 13.
The husband and wife, aged 55 and 54, who live within the Trafford borough of Greater Manchester, had denied all charges against them.
During the trial the jury was played video police interviews of the younger girl, who initially raised the alarm by telling her teacher last January.
She said Nelson had been “doing rude stuff to her” at his flat as he minded her after school while both parents were at work.
Nelson also played pornographic videos on his computer in her presence and masturbated to them.
When she returned home she told her parents she had been assaulted and was sore.
“My dad said if he is hurting you then don’t go down there, but I said if no one is in (at home) then I have no choice,” she said.
“My mum said they could not do anything because he helped them out with money and a car.”
Molloy raped her under her own roof.
Again both parents were informed but no action was taken.
The couple were found guilty of several counts of child cruelty, while the husband was convicted of various child sex offences involving the older girl.
Sentencing of the couple, who cannot be named for legal reasons, takes place on October 1.