A Melbourne mother who helped her nephew rape her 12-year-old daughter could walk from prison in less than a year.

The woman was in a sexual relationship with her adult nephew when her daughter was first assaulted in 2011.

They forced the young girl to watch them have sex, the nephew touching the child as it happened.

He then decided he was going to "break" his 12-year-old cousin's virginity.

The girl was crying and telling her cousin to stop because she was in pain.

Her mother, aged in her 50s, held her child's leg for a better view.

The woman was sentenced to four years jail with a two-year non-parole period on Friday.

She will be eligible for release in less than a year because she has already served about 450 days.

Her nephew was handed nine years behind bars and must serve at least seven.

The man, aged in his 40s, continued abusing his young cousin for three years after the first rape, though the girl's mother wasn't involved again.

It was usually when the girl was alone with the nephew.

The judge said the pair's crimes against the girl were a gross breach of trust.

They both exploited the position of power and authority they had over the-then 12-year-old, she said.

The girl suffered nightmares, flashbacks and social problems as a result of the abuse she endured.

Her victim impact statement made for "disturbing reading".

"You have taken away her childhood and innocence," the judge said while sentencing the pair.

The court heard the girl's mother was overwhelmed with guilt, shock and shame over her crimes.

An expert believed she was feeling controlled and intimidated by her nephew, and suffering from a depressive disorder at the time of the offending against her daughter.

The woman pleaded guilty to sexual penetration of a child and an indecent act with a child.

Her nephew pleaded guilty to persistent sexual abuse of a child under the age of 16.

He had shown some remorse, but had limited insight into his crimes and would require extensive sex offender treatment to reduce his moderate risk of reoffending, the court heard.