Isabelle Welsh was a "happy and thriving" child before her mother, Alexandra Walker, invited Harrison Simpson into their lives in Thornaby, Stockton, in June 2025, Teesside Crown Court heard.
A judge said Isabelle was the victim of a campaign of violent and sexual abuse at the hands of those meant to love, care for and protect her.
Simpson, 22, and Walker, 26, were both found guilty of murder and child cruelty, with Simpson also convicted of sexual abuse of a child. Simpson must serve a minimum of 28 and a half years and Walker 22 years.
Ms Justice Norton said both were "equally and jointly responsible" for Isabelle's murder and the campaign of cruelty she endured, with the couple putting up a "wall of silence, lies and obfuscation" about the last weeks of her life.
The court heard Isabelle was fatally injured at her home on the morning of 13 September 2025, with Walker not calling 999 until hours later.
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