The judge said Sarah Walker “was the guiding mind behind the detail of [the] plan” to cover up Bernadette’s death.
Scott Walker told jurors Bernadette’s allegations of sexual abuse were “untrue”.
After telling her mother about the alleged abuse, Bernadette was sent to stay with Scott Walker’s parents.
When he collected Bernadette, his phone was switched off between 11:23 and 12:54 BST, the court heard.
Prosecutors said it was during these 91 minutes that he killed his stepdaughter.
When his phone reconnected to the network, the first call he made was to Sarah Walker, which lasted nine minutes.
The pair “concocted” a story during this time to cover up her death, the prosecution said.