Maurice and Patty Leach, who identify themselves as the uncle and aunt of Josh Powell, said the family tragedy was the result of a series of failures, KIRO-TV, Seattle, reported Wednesday.
The couple issued a statement blaming the tragedy in part on "the various questionable government agencies' practices, religious bias, the Internet kangaroo courts, and sensationalized news media, all of whom have circumvented the laws that protect all our rights to due process."
Powell left a voice mail for his family saying he could not live without his sons just 20 minutes before an explosion rocked his home in Puyallup, Wash.
He was a suspect in the 2009 disappearance of his wife, Susan Cox, and had been ordered to undergo psychosexual testing before he could regain custody of Charles, 7, and Braden, 5.
When the boys arrived for a supervised visit Sunday, Powell would not let their caseworker into the house.
In a recording of one of her 911 calls you can hear a child crying inside the house, the Tacoma News-Tribune reported.
Detectives said Powell meticulously planned their deaths, giving away all their toys and books to Goodwill and purchasing 10 gallons of gasoline to spread throughout the house.
He used a hatchet on the boys before setting the house on fire, the report said. All three died of carbon monoxide poisoning.