The pair then cleaned themselves up, had sex, ate ice cream and watched the movieTwilight.
Kim and Lucas, who are Britain's youngest double-murderers, were quickly dubbed the 'Twilight Killers'.
Now, their chilling confession tapes have been released, in the new documentaryMurdered By My Daughter which aired overnight in the U.K.
In her confession, Kim says she felt "relieved" after killing her mum, Elizabeth, and sister, Katie.
"My mum does not have to deal with me anymore being suicidal, and she does not have to wake up worrying every morning to see if I am still alive," she says, as reported by The Sun.
Kim explains she had Lucas kill her 13-year-old younger sister so that she didn't have to go "through all the heartbreak and like just all the emotions and stuff" of losing her mother.
"Basically me and Kim... she knew I hated her mum, and I said 'I wish I could kill her' and she said 'yeah,'" Lucas said.
"She thought I was joking but I was being serious. Then she realised I was being serious so she started being serious."
The documentary alleges that Kim and Lucas were planning to commit suicide after the murders, but that Kim couldn't go through with it.
"I went into her mum's room and stabbed her in the neck while she was asleep on her side and smothered her face with a pillow," Lucas told police.
"And after I knew she had gone, I went into Katie's room - which is the same room as Kim's - and I thought I stabbed her, but...I thought I stabbed her, but I’m not a hundred percent sure. It was, like, her on a mattress and then I smothered her face with a pillow too."
Elizabeth's oldest daughter has since said she's disgusted at the Channel 5 documentary, telling The Daily Mail that it drags her "mum's name through the dirt."
This article first published on Marie Claire.