A meth-addicted mother who encouraged her husband to sexually abuse their six-year-old niece and film it for the pair to watch together, has been jailed for five years and one month.

Sarah*, 26, and her now-estranged husband John*, 30, abused their niece, whilst the girl and her mother stayed with the couple in 2014.

The uncle and aunt of a six-year-old girl abused by the pair have both been jailed.

During Sarah's sentencing in the Perth District Court on Friday, Judge Anette Schoombee told Sarah she had abused the trust her niece and sister had placed in her, after involving the young girl in her and her husband's meth-fuelled sexual fantasies.

The court heard Sarah and John started their offending when Sarah offered to begin bathing her niece.