The mother of two boys accused of raping their sister said she rang social services after a talk with her children, Birmingham Crown Court heard.
The woman said she contacted officials when her daughter, now aged seven, said her brother had touched her indecently.
She said her 13-year-old son also told her that he had seen his older brother lying on top of the girl.
The boys, aged 13 and 15, from Birmingham, deny raping and assaulting their sister between 2006 and 2009.
The mother told the court: "I can't remember her exact words.
"She didn't say what had led up to it- she just said he (the older brother) would ask her to allow him."
She told the court the girl had made the revelations in a series of talks in the space of a week last summer.
An older sister of the alleged rape victim told jurors last week she had been assaulted by the older brother at home and in the home of a relative.
The 15-year-old brother denies two charges of rape and four charges of sexually assaulting a child, his brother, 13, denies two counts of rape and two counts of sexual assault.
The trial continues.