Leigh Turner, 54, and Anita Turner, 51, have each been jailed for 27 months for perverting the course of justice.
Elliot Turner, 20, was recently found guilty of strangling Longley, his girlfriend, at his parents’ home in Dorset.
Covert police recordings caught Mr and Mrs Turner discussing their disposal of the confession letter.
Handing down the sentences, Mrs Justice Dobbs said: “I appreciate it must be very difficult for a parent when faced with the situation you found yourself in but you well knew what you did was wrong and against the law.”
She added that destroying a confession letter was a “very serious offence”.
Mrs Turner also removed a jacket from the crime scene for fear it would implicate her son.
Outside Winchester Crown Court, Emily’s grandfather, Ron Longley, said: “Today brings a sort of conclusion to a dreadful year.
“We are satisfied with the sentence, we feel it was balanced and just.”