The probe revealed how the parents had left three babies for six hours while they went boozing - only checking in on them occasionally.
Authorities were aware mum and dad both struggled with alcohol, but early interventions were stood down as the mum refused permission for help.
The tragic event unfolded during a four-day holiday, for which the parents had taken their six children to a resort just 35 miles from Wigan, Greater Manchester.
Parents known to social services
Both the mum and the dad, a violent offender released from a 30-month jail stint for robbery just before the relationship started, were known to social services.
The authorities were aware of reports of potential domestic violence after the mum turned up at her nursery job with two black eyes.
Staff at A&E also reported her after she turned up drunk at the hospital when 20 weeks pregnant with her first set of twins.
At the time of Child M's death, the dad was described as 'unsupportive' and would often go off 'for days at a time on drinking binges'.
The doomed holiday included three children - all primary-school age and younger, a 13-month-old child and the 10-week-old twins.
At around 10am the dad found his youngest daughter unresponsive in a baby car seat in the hotel room next to her twin sibling.
Twenty minutes later the dad carried his lifeless infant down to the hotel reception where paramedics arrived after just two minutes.
Paramedics noticed signs of rigor mortis, which sets in as soon as four hours after death, and the heartbreaking decision was made to 'discontinue resuscitation'.
She was pronounced dead at 11.23am.
The report states: 'Child M's death occurred in the morning, following the second night of the family's holiday.
'The three youngest children were settled for the night in the attic bedroom anytime between 7pm and 7.30pm the previous evening.
'The twin infants were placed to sleep in their car seats which were upright on the bottom bunk bed.
'The parents had planned to use a sleep system which included carry cots, but this was reported to be too heavy to carry up the three sets of stairs to the attic room.
'Child M's parents and their three older children then went downstairs to the hotel lounge to socialise.
'The parents were reported to have been checking on the three younger children every 30 minutes.'
Alcohol found in hotel room
Police, who later carried out a criminal investigation, were able to verify the parents had checked on their children.
According to the parents, they returned to the bedroom at around 1am after drinking in the hotel bar, and Child M was fed by her mother at 2.30am.
After the tragedy, blood tests were taken from the parents after police found 'empty cans of lager and beer bottle' in the bedroom.
The report revealed how the couple's both sets of twins suffered a series of health problems.
Of the first set born in June 2015, one child died after three days and the other suffered from 'complex needs'.
Child M was also considered to be 'high risk' who needed to be resuscitated at birth and weighed just 920g.
She remained under the care of the Neo-Natal Outreach Team (NORT) who 'worked with the family to prepare for their holiday'.
The following review into Child M's death read: 'NORT members believed the children would be sleeping in 'carry cots'.
'But the parents did not follow through with this original plan as they were 'too heavy' to carry up the three flights of stairs to the family bedroom.'
Describing how the family had four referrals to child social care in a 10-year period, the reviewers stated: 'Mother's use of alcohol was a repeated focus of concern.
'There was some evidence that professionals attempted to work with the parents to assess their alcohol use.
'There were challenges for professionals in understanding how they could more effectively work with mother [who] minimised her alcohol use.'
At the time of Child M's death, the family were already subject to a section 17 intervention, which defines a child as being 'in need'.
Child M was already considered at 'high risk' of sudden infant death syndrome, due to being a premature baby from a 'multiple birth' in a household where her main caregivers smoked and drank.
The review added: 'The review has identified areas of multi-agency practice that could be strengthened.
'But it has not identified any serious omission in practice that contributed to the death of Child M.
'The parents could not follow through on the plans to ensure Child M could sleep safely in the hotel and made the choice to place Child M to sleep in a car seat.
'This is one of the most significant risk factors in sudden infant death.
'Depending on the source of information, parents are advised to remove their child from a car seat every 30 to 130 minutes to prevent sudden death.'
The report found that although certain areas of multi-agency practice need to be bolstered, there were no 'serious omissions in practice' which caused Child M's death.
No criminal charges were brought following the investigation by Greater Manchester Police.
Following an inquest, the coroner reported a narrative verdict, saying: 'Having been fed at 2.30am, Child M fell asleep while secured in a car seat which was placed in an upright position on a bunk bed in a room of the hotel..
'Shortly after 10.20am, later that morning the infant was found deceased and still seated in the upright car seat.
'Despite a subsequent forensic post-mortem, it was not possible to ascertain the cause of death.'
Dr Paul Kingston, the Independent Chair of the Wigan Safeguarding Children Board, said: 'This is a truly sad loss of a child and we send our continuing deepest condolences to the family.
'The findings highlight the difficulties faced by families in sustaining safe sleep arrangements, amidst gaps in cohesive professional advice from many sources, not least in relation to sleeping in car carry seats which is not a unique issue to Wigan.
'The commitment of the services that supported the child and family in the years preceding the child's death was unquestionable.
'The reviewers have identified many examples of good practice by professionals in providing information and support.'
