Teresa Robinette has spoken exclusively to Daily Mail, revealing that Louise Turpin was just about to turn 40 when she hit a midlife crisis.
Ms Robinette, and her half-brother Billy Lambert, have speculated that the now infamous couple's 'perfect life' went horribly wrong when the two suddenly decided to 'let loose.'
'Louise had never drunk a beer or smoked a cigarette or done a drug or anything like that in her whole life,' Teresa told the publication.
'Then one day, just as she was turning 40 she called and told me that she and David were going to have a drink.
'When I spoke with her later that night she was drunk for the first time in her life. I was in shock.'
Teresa tells Daily Mail that she believes the horrific events that the couple allegedly inflicted on their 13 children was a result of Louise and David's 'later-life' experiments.
Teresa further explains, 'At that same time she started gambling, she started taking trips down to Vegas and she got a totally different hairstyle — dramatically different.'
Ms Robinette also spoke to Megyn Kelly Today, telling she believed the couple's bizarre sexual fantasies were a catalyst for the traumatic events.
Teresa told the Megyn: 'At the time, she said...that the older children were helping take care of the younger children so her and David could sow the wild oats they didn't sow when they were younger.'
Both David and Louise have pleaded not guilty to nearly 40 offences, including torture, false imprisonment and child abuse.