Trinity Culley, then aged nine, grabbed towels and delivered her sister while the family waited for an ambulance at their home in Fingringhoe, Essex.
Mum Dee said Trinity was "amazing but a bit embarrassed as she saw everything".
"We're certainly not going to have to have that 'where babies come from' talk with her after this," she said.
Mrs Culley, 28, went into labour two weeks early on 31 March.
"Trinity dashed downstairs and grabbed towels and covered me up," she said.
"She just said, 'It's alright mum, I've seen this on One Born Every Minute'."
Trinity was not allowed to watch the programme but had been doing so in secret in her room, Mrs Culley said.
Jasmin Elizabeth-Rose weighed in at 7lb 1oz (3.2kg) and arrived "in about five minutes".
Mrs Culley said she and husband Terry, 34, were "extremely proud" of their eldest daughter.
Trinity, who has since had her 10th birthday, was "chastised" for watching the show on her television "which was not meant for that sort of thing".
Mrs Culley said: "She just turned to me and said, 'Well mum, if I hadn't watched it, I wouldn't have known what to do, would I?'."
A girl delivered her mother's baby after "seeing how it was done" while secretly watching hospital television series One Born Every Minute.