I've always believed she did it, and she did it alone. This was NOT just circumstantial evidence. She was a heartless bitch, shooting silly string and yelling HAPPY BIRTHDAY to her 2 dead children. How the hell could they have a Happy Birthday if they are dead I ask you?

And the standard in the US is beyond a REASONABLE DOUBT, not beyond any doubt.
From Tru tv Crime Library on Darlie Routier:
" In the kitchenette, something very telling had occurred. Sgt. Nabors thought it was strange that the sink was spotless and white while the floors and edges of the countertop around and above it were blood-smudged. And if someone had taken the effort to clean the sink of blood — why? His job being to process blood traces at a crime scene, Nabors went to work.
"(He) conducted a test to detect the presence of human blood that cannot be seen with the naked eye," explains the book, Precious Angels. "The chemical compound Luminol is the tool that investigators use for this test. If the white crystalline compound in the Luminol detects the copper component found in human blood, the area sprayed becomes luminescent, casting a brilliant bluish light. The sergeant sprayed the sink and the surrounding counter. When the lights were switched off, the entire sink basin and the surrounding counter glowed in the dark."
Repeating this process on the leatherette sofa, the detective found a small child's handprint glowing iridescent blue near the edge where Damon had been stabbed. Like the blood in the kitchen, someone had wiped it away. Again — why?"
Crime Scene Tells Story
Returning inside, he followed the bloody footprints. They indeed led from the room where the children were slain through to a utility room then onto the concrete floor of the garage, trailing off below its window. But, again, the screen seemed an unlikely escape port. Doubling that suspicion, the dust on the sill was undisturbed, there were no hand prints, bloody or otherwise, around the window; odd, since the killer in forcing his way through the window would have had to hang onto the walls for balance!
The investigator double-tracked to the yard, this time looking for drops of blood left behind by the slayer in flight. Surely, his savagery had produced vast amounts of blood and his clothing would have been dripping with it — yet there were no apparent traces beyond the interior of the house. Not on the mulch below the window, not on the yard's manicured lawn, not along nor atop the six-foot high fence that surrounded the yard, not in the alley. The blood was contained within the house. Nowhere else.
In the entertainment room where Darlie described a struggle, Cron found little evidence of a melee having taken place. The lampshade was askew, and an expensive flower arrangement lay beside the coffee table. Nothing more out of place. He found, in fact, the fragile stems of the flowers unbroken — as if the arrangement hadn't fallen, but been placed there.
In the kitchenette, only Darlie's bloodied footprints were visible. Pieces of a shattered wineglass, too, lay among the prints, and a vacuum cleaner had been deposited on its side. Blood underneath these items indicated, to him, that they were dropped after — not before, nor during— the violence.
Atop the kitchen counter sat Darlie's purse, which appeared in order and undisturbed, and several pieces of jewelry — rings, a bracelet and a watch — aligned in order, untouched.
Reports author Barbara Davis in Precious Angels: "Everything the professional saw at the crime scene disturbed him. The lack of a blood trail away from the home coupled with virtually no signs of a struggle bothered him most."
Late afternoon, after his thorough and all-day examination, he summarized his findings for Lt. Jack and Sgt. Walling. "We all know the crime scene tells the story. Problem is," he nodded, "that story's not the same one the mother's telling. Somebody inside this house did this thing. Gentlemen, there was no intruder."
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I believe the crime scene findings tell it all. And this next part from The Trauma Room tells the rest of it:
Almost immediately, the hospital's personnel sensed something amiss with the mother, for while she outwardly seemed agitated by her tragedy, repeating over and over "Who could have done this to my boys?" her reaction struck them as insincere and artificial.
Trauma nurse Jody Fitts, an RN for eight years, recalls, "Darlie was wheeled by Trauma Room 1, where her dead child was. She glanced over there, and I was very concerned she would get more upset. His physical condition alone was disconcerting. He was nude and covered head to toe in blood. Tubes were still held in place with tape, and brown bags had been placed around his little-bitty hands to preserve any possible evidence. It was a very stressful and horrible sight...I'll never forget it. (Darlie) saw him. She had absolutely no response, just turned her head back and stared straight ahead — cold as ice."
And this:
Of Shadows and Silly String
Darlie Routier had not yet returned to her home on Eagle Drive since that horrible morning; she, Darin and baby Drake had been staying with Mama Darlie in Plano. Needing some articles of clothing, she telephoned her friend Mercedes Adams a few days after the funeral to ask if she would mind driving her there. Mercedes complied, but expected Darlie to buckle under upon walking into the place that took the lives of her two sons. The girlfriend was in for an awakening.
Death lingered in the foyer, but Darlie, Mercedes noted, charged onto the scene seemingly unaware and like a bull elephant, arms akimbo, shouted, "Look at this mess! It'll cost us a fortune to fix this shit!"
"Right there where her boys were killed, and that's the first thing she said to me. I put my hands on Darlie's shoulders and said, 'Darlie, look me in the eye and tell me you didn't kill the boys.' She looked me in the eye and said, 'I'm gonna get new carpet, new drapes, and fix this room all up.' I couldn't believe it."
...................
Brantley reported other observations. "For a violent struggle to take place as the mother claimed, no real breakage occurred. After looking at the crime-scene photographs, it appeared to me that the intruder who committed this crime had a strong connection to the material items in the home. The living room was fairly small and compressed. Two adults fighting would have resulted in a lot more broken things. A lot of fragile items in the living room that should have taken the brunt of a struggle were not broken."
His conclusion: Damon's and Devon's slayer was someone who knew them and knew the premises. The entire scenario had been planned in advance — and staged.
She did it! She was convicted of it! And the cold hearted bitch is in prison for it! Enough said!