The killer, then a salesman with Swindon-based kitchenware firm Kleeneze, is believed to have spent the night in the house before making his getaway in disguise.
Cotterell had denied the murders since being arrested in April last year, but changed his plea as his trial was due to begin at Norwich Crown Court on Monday.
Police - who offered a £25,000 reward in the hunt for the women's killer - do not know why Cotterell committed the murders, but said he was "clearly very, very evil and very wicked".
The killer, then a salesman with Swindon-based kitchenware firm Kleeneze, is believed to have spent the night in the house before making his getaway in disguise.
Detectives believe the women's bodies had lain in the cottage for several days before they were found by neighbours. More than 20 whippets owned by Janice were also in the house.
The pair suffered multiple stab wounds. Janice had been bound with black masking tape.
A witness noted the number of a suspicious car parked outside the Sheridans' cottage, it was later found to be a near match to Cotterell's.
Another witness remembered seeing a similar green car driving away from the house with a man wearing a "Bobby Ball" wig at the wheel.
Bloodstained bag
Cotterell's fingerprints and a shoe print matching one of his boots were found in the house.
His DNA was in the wounds on Janice's chest and police later found a bloodstained Kleeneze bag at Cotterell's home containing both Janice and Connie's DNA.
Cotterell admitted meeting Janice and Connie in 1998 when he was working for a home improvements firm.
He told police that he had sold them a window but had never been in the house again.
'Terrible and terrifying'
But detectives have no idea of the killer's motive.
He had convictions for two house burglaries in 1983. Detectives said there was evidence to suggest that he had an obsession with a teenage girl who lived at one of the houses.
Passing sentence Mr Justice Owen told Cotterell: "These were terrible and terrifying offences.
"How they came to be committed is not clear but that they were committed and committed by you is abundantly clear. There is only one sentence."
Janice's half sister, Diana Penfold, who lives in Wisbech, Cambs, added: "I was quite dumbstruck when I heard that he was pleading guilty.
"We are just devastated and we just want to know why. We want to know why and we need to know.
"We were hoping that we might get some answers during the trial but we still don't know. The last year has just been unimaginable."