Salim Mehajer's sister Fatima is facing a potential prison sentence after pleading guilty to a raft of electoral fraud offences, while her brother is on trial for seeking to rig the Auburn council election that catapulted him into public office.

On the eve of the siblings' criminal trial in the Local Court, Ms Mehajer struck a deal with Commonwealth prosecutors to plead guilty to 77 counts of giving false or misleading information to the Australian Electoral Commission.

Clutching her baby daughter, Ms Mehajer appeared briefly in court on Thursday to confirm the deal before being excused by magistrate Beverley Schurr.

The offences, under the Criminal Code, carry a maximum penalty of 12 months' imprisonment.