Christina George-Harvan, 30 and Katelyn Francis, 21, were celebrating at Jimmy K's bar in New Brighton, Pennsylvania, after the ceremony on April 24, 2014 when they started arguing over who should drive home.
The row spilled into the parking lot and then into new husband Jeremy Harvan's car when the gun went off.
On Friday a jury convicted her of involuntary manslaughter and reckless endangerment - both class one misdemeanors.
Her bond was revoked and she was led back to the jail cells in tears - but her family say they are pleased with the outcome.
Her mother Julie Francis told WTAE: 'I didn't think there would be a not guilty verdict. We figured there would be something. And this is the least of those charges
[...]
George-Harvan, a former nurse, took to the stand on Thursday during her trial. She told the court about the hours leading up the shooting - saying there was no tension with her niece.
According to CBS Pittsburgh, she said: 'I was happy…she was so cute. We were only nine years apart. The first thing she did was say, 'Aunt Tina, I'm so happy for you,' and gave me a big hug.'
As the night went on they drank more, the court heard, and soon Mr Harvan was determined to drive home.
However Francis demanded she be the designated driver and tried to take the keys off of him.
George-Harvan said: 'When I turned around, the glove box is open and I see this gun. I think I gotta move this gun. I go get it out of the glove box to move it and boom the gun goes off.
The defense attorney then asked: 'Did you squeeze the trigger?'
And she replied: 'No.'
Recounting the moment she realized she had shot her niece, she said: 'Everything freezes, Jeremy says, 'You just shot your [expletive] cousin'.
'He steps over her dying body and he runs. I put pressure on [Katelyn's] wound and I screamed and dialed 911.
She added, 'I felt devastated. It was like a nightmare. Your wedding day is supposed to be the best day of your life and my niece is dead.'