Arthur McElhill, Lorraine McGovern and their five children died in the blaze in Omagh in November 2007.
Mr McElhill, a registered sex offender, was suspected of starting the fire.
A Public Prosecution Service spokeswoman said: 'No prosecution will be taken in relation to the fatal deaths at 4 Lammy Crescent.'
Inquests are expected in August or September.
Neighbours battled unsuccessfully to save the family after the estate home erupted in flames.
An independent report by Henry Toner QC pinpointed deficiencies in the way agencies communicated Mr McElhill's record and made 63 recommendations.
The inquiry criticised the way information was passed on within disciplines of the Western Health and Social Care Trust and other agencies and the assessment of potential risks posed by Mr McElhill to teenage girls by reason of his sex offences.
Police removed a teenage girl from his care weeks before the fire.
