Bride-to-be Grace Dyson was walking in the rain with partner Jason Booth when the tip of his brolly got caught in a crevice in the wall as he let it down.
Grace, an apprentice administrator, was knocked by the violent jerk and stumbled into the road.
But she fell into oncoming traffic at the same second her uncle Ivan Beaumont was passing in his van on the road in Holmfirth, West Yorkshire, last August.
An inquest heard the chance accident left Grace, 18, a talented linguist, with “non-survivable head injuries” as the split second knock threw her under the Nissan cabstar.
Mr Booth, who said he and tragic Grace were “inseparable”, told Huddersfield Coroner’s Court the accident happened in a fraction of a second.
The chemistry student said: “We had decided to go to the chippy so we were walking down the road towards it at the time. It was raining quite badly.
“It was quite a big umbrella with an elongated handle. When we set off the brolly was up. On the side of the pavement we were walking down there was a dry stone wall - I was nearest to the wall and Grace was on my left.
“The metal end of the umbrella got stuck in a crevice in the wall. It made me jolt to the left hand side - I remember seeing Grace’s foot slip over the edge. I think that’s when she fell.”
He added: “It happened in a split second.”
Mr Beaumont was travelling home from a job in Pocklington, North Yorkshire, when he hit Grace. She was later pronounced dead at the scene by emergencies.
The labourer told police in a later interview he had driven that way especially to stop off at a nearby co-op, but decided against it due to the rain.
At the split second Grace stumbled, he told police he said he had just spotted his niece and was about to try and catch her eye to wave.
Detective Sergeant Noel Lowden from West Yorkshire Police told the court how Mr Beaumont recognised the pair when she “wobbled and slipped, like when you turn over on your ankle” fractions of a second before he hit her, adding he had “no time to brake”.
Det Sgt Lowden said: “He had no time. She fell in the road and hit her with his van. It all happened so quick and she went under the wheel”, adding “it’s a tragic set of circumstances that Grace lost her footing at that exact time.
“Sadly this proved fatal. The chances of this occurring are unbelievable.”
Mr Beaumont was questioned by police but no charges were ever brought.
Eyewitness Lesley Hardy, who was driving on the other side of the narrow road at the time said he stopped as soon as he saw the shocking split-second collision.
In a statement read in court the driver said he had spotted a male dressed in gothic clothes and a “black coat” in the road.
“I realised this was not a back coat, but a woman. My wife and I got out of the car and went over to where she was.
“The male was very disorientated and seriously distressed. There was no movement or sound from the lady in the road.
“Her body was sort of twisted. I spoke to the passenger who said ‘it’s his niece’ - I couldn’t believe what I had just heard.”
Recording a road traffic accident verdict, Assistant Coroner Mary Burke said: “No one could have anticipated that this was going to happen. No one could have changed the pattern of events events.
“It happened in fractions of a second. There was no way Jason would have predicted what was going to happen. I offer my deepest condolences to you.”