The independent inquiry into child sex abuse in England and Wales is looking at "tens of thousands of pages" of information about any potential failures to protect children in the care of the Nottinghamshire Councils alone, its counsel said.
The scale of the task of officials currently trawling through information, including material from councils, police, the Crown Prosecution Service and the Health Department, "cannot be underestimated", Inquiry counsel Patrick Sadd said.
He told a preliminary hearing of the Independent Inquiry into Child Sexual Abuse (IICSA): "To date this investigation has received tens of thousands of pages of which a large proportion have already been reviewed in some form.
"This means that we are considering the contents, deciding upon their relevance and using them to inform our decisions about the approach that the investigation will take going forward."
An insight into some of the alleged suffering was given as the central London hearing was told of two brothers who say their sister was raped and abused while they were in foster care.
Caoilfhionn Gallagher QC said the brothers, who can only be named as D18 and D19, were not interfered with but they witnessed the sexual abuse, fondling and tears of their sister.
She said the sister, named only as D17, was left in "that oppressive environment" while her brothers were later removed.
Ms Gallagher said D18 has post-traumatic stress disorder while D19 is "particularly vulnerable and traumatised".
They are fighting to have their voices heard as a key interested party, or core participants, in the inquiry.
Ms Gallagher argued they should be considered as core participants as they have been "indirect victims of the harms suffered by their sister".
The inquiry is to look into the extent of any institutional failures to protect children in the care of the Nottinghamshire Councils from sexual abuse and exploitation.
The IICSA's investigation was launched after three police investigations in Nottinghamshire historical abuse allegations in children's homes.
They included Operation Daybreak which opened in 2010 and Operation Xeres which was launched in 2014.
Operation Equinox, which opened in 2015 and absorbed the previous two investigations, is ongoing, according to Mr Sadd.
There has been one conviction of a former children's home employee and two other convictions relating to child abuse.
Mr Sadd said some of the information received is about the prosecution of Andris Logins who was convicted of indecent assault and rape at Beechwood Children's Home between 1980 and 1985.
Logins, who was aged 22 to 27 when he abused the children, was jailed for 20 years in March 2016.
There are 75 core participants, including 69 complainants, to the inquiry so far.
Decisions on remaining parties who have applied to be part of the inquiry will be made at a later date.
The Nottinghamshire arm of the inquiry is one of 13 areas of public life being scrutinised for child protection failings, ranging from the army to the Roman Catholic Church.
Carolyne Willow and Anna Sains, who between them have 70 years social work experience, were described by their lawyer David Enright as some of the best placed people to help the inquiry understand how the institutions worked and to challenge official positions of what went wrong.
Arguing for them to be allowed core participant status, Mr Enright said: "(They were) professionals on the coalface who were wrestling with the institutions at the time, trying to protect children at the time, who were whistleblowing about the abuse at the time, who were demanding investigations at the time and who were themselves being misled at the time and thereafter."
Ms Gallagher told the hearing their application was "very strongly supported" by one of her clients who is an alleged victim.