Shannon Cox dialled 999 when her mother suddenly went into labour in the bathroom during the night.
She carried out instructions given by a call handler and within minutes had delivered a baby boy.
She has now been presented with a "Superhero" certificate by the Welsh Ambulance Service.
After safely delivering her brother in the early hours of 4 March, Shannon gathered towels to wrap him in and even used a shoe lace to tie his umbilical cord.
An ambulance crew arrived shortly afterwards to make sure mother and baby were safe.
As a reward for her bravery, she was allowed to choose the new baby's name - Riley Henry James.
Mum Sarah Cox, 30, said: "It was just me and the kids at home. I started having one or two little pains, nothing major, then went to run a bath and that's when my waters went.
"Shannon and my two younger ones were home. I woke her up and she phoned for the ambulance, and gave them the details.
"They did ask me to get out of the bath, but I couldn't manage it so she followed all the instructions they gave her over the phone and before I knew it I'd given birth.
"She did brilliantly, she didn't question anything and she just got on with it. The team that came were also brilliant and they couldn't praise her enough for what she did."
Call handler Danielle Burrows, who has helped to deliver six babies over the phone during her two-and-a-half years with the ambulance service, said: "She was as good as gold throughout the entire call.
"I could hear Sarah in the background so I was talking to them both, but obviously she had to do a lot of work in preparing her mum for birth.
"There was a lot of commotion, then the next thing I know her mum is on the phone saying the baby's been delivered."
Mrs Cox added that Shannon and three-month-old Riley have developed a special bond.
"She can't do enough for him and the first thing she does is check if he's okay. She's so loving towards him," she said.
"I don't think Shannon realises just how important what she did was. The only thing she says when people ask her is 'That's what the midwives do', as if it was nothing."
An 11-year-old girl from Argoed, Caerphilly county, has been praised for her bravery after helping to deliver her baby brother.