I agree with fusionconfusion re "But if nothing is being reported and logged there's never going to be a case built, is there?"
Also very good points from niceday and curlywurly4.
Anyone can report suspected neglect or suspected abuse. They can do so anonymously.
I am so sorry for your experiences WeveGotAHomelessLove and hope you have found some peace and justice in the last few years.
and 
NeedsAsockamnesty re "Secondly if a 11 and 9 year old are being left alone while their sole carer is in the pub, then it's standard procedure to call the police to conduct a welfare check. Parents have been arrested and prosecuted for this, it comes up time and again in child protection conferences."
And
"It would not fit the criteria for prosecution unless the children being unsupervised left them at risk no matter the reason why they are alone."
The Op said in her opening post "they cannot be trusted to be left in a room as they play fight and rip up paper (letters etc) or knock into things and break stuff or will pick up a load of clean laundry and throw it round the room."
I know what she describes is not exactly dangerous behaviour but I also feel that a nine year old and an 11 year old might not be safe to be left.
-What if they are hungry and start a fire trying to cook something?
-What if the play fighting turns into real fighting?
-What if the knocking into stuff results in an injury?
These are not two regular 9 and 11 year olds, they are two children who the OP said "DSDs just do not seem to be growing up since I met them 7 yrs ago." Does that really mean they are somewhat like a two year old and a four year old! I hate to sound so alarmist but being left alone unsupervised (potentially for long periods of time) can be very frightening for children.
I am not sure the aim needs to be to successfully prosecute the children's birth mum, more to make her address her behaviour.
Re It would be unusual for something like this where a neighbour has agreed to be contactable (and possibly also checking) and the parent is not gone for several hours to get that far. Did the OP say that there was a neighbour on alert to look out for the girls? (Genuine question).
Needs re "It might just be the reason why some posters are not happy to instantly yell sexual abuse or neglect in these situations." The OP has said herself she thinks there is neglect. And she is the only person here who actually knows the girls! Everyone else is just responding to what the OP has written.
I am not sure any person here has said that there is definitely sexual abuse, just there might be. The other factor, of course, is there may have been sexual or other abuse in the past, and this is the fallout from that behaviour. So it may not be ongoing but was there in the past, maybe. But even if there is no sexual abuse (and I hope to God there is not, of course) there is neglect.
The OP herself has said she is worried the older child will be 'eaten alive' once she reaches high school later this year.
My own dd goes to high school later this year. The idea that I would have that fear but be unable to alter anything, is awful.
But I really do not think the OP is powerless as anyone can report 'suspected' neglect anonymously.
Not everyone has jumped on the OP or accused her doing nothing.