Francesca Goodby from Bearwood first sprang into action when her mother Kay gave birth on the bathroom floor two years ago.
The cool-headed youngster, who was just six at the time, helped deliver 6lbs 8oz Roisin by encouraging Kay to push after remembering an episode of Casualty.
Incredibly, the schoolgirl helped Kay give birth again after she went into labour in August this year.
Kaycalled the midwife after her waters broke and Francesca sat with her while father Michael, aged 35, looked after two-year-old Roisin downstairs.
But before the midwife arrived on August 12, Francesca delivered her baby sister Saoirse who weighed a healthy 6lbs.
Mini-midwife Francesca fetched towels and even held baby Saoirse's head when she was born.
Her proud mother Kay, 30, said: "We're so very proud of her.
"She was the calmest person in the house, she kept telling me to stay calm and helping me to breathe.
"And even when we realised the baby was being born the wrong way she was able to hold the head and keep her safe.
"Her dad had to look after Roisin in the other room so Francesca was shouting through to tell him what was going on, she would tell him when the head was out an if everything was OK.
"Michael was on the phone to the 999 operator relaying the information Francesca was telling him so it was a team effort.
"She learned the first time how to deliver babies from watching Casualty and ever since I fell pregnant again she has been watching it with me and also the show One Born Every Minute so she knows what to do.
"The midwives did arrive eventually, but Saoirse was already out. They cut the cord and finished off cleaning the bathroom.
"One of the senior midwives is so impressed with Francesca she said that if she needs a job I should contact her and she will get her a place on the training course. We joke that Francesca is probably the youngest midwife in the country."
Kay had been experiencing false alarms for several weeks, and visited the hospital just hours before the birth.
When her waters broke at home she phoned the NHS helpline and was told the midwife would be on her way.
But before the midwife arrived, she gave birth.
Francesca, who wants to become an artist when she grows up, said: "As soon as mummy said she was pregnant I was preparing just in case it happened again.
"I just had to stay calm and keep bringing towels and keep telling daddy what was going on.
"Then I held the head while she came out. I was excited and glad to help."