Melanie Smith will be sentenced on 8 May for murdering five members of the same family in a flat fire in Prestatyn.
Mr Justice Griffith-Williams warned the 43-year-old that she should expect a life sentence.
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A wealthy couple claimed more than £43,000 in benefits while having a small fortune in the bank, Cardiff Crown Court has heard.
Michelle Smith was found guilty in 2012 of murdering her six-week old daughter Amy, by dosing her with adult painkillers.
Welsh family beaten unconscious by gang in Thai tourist resort. The grandparents and their son were punched and kicked unconscious.