After the first shot, which struck Lauren on the side, the teenager heroically tried to run across the kitchen to shield her sister from their attacker.
The second shot hit Lauren in the chest. As she fell to her knees McNally turned his gun on Brenda.
Her right arm and side took the full force of the blast.
The gunman then calmly walked from the house and slowly drove from the scene of carnage he had left behind.
Inside Brenda, gravely wounded, crawled across the kitchen floor to help her dying sister.
Her two children screamed hysterically as she tried to revive Lauren.
"When I reached her all I could smell was warm blood and I just fell down beside her," she said.
"I started talking to her and shouting at her but she was un-conscious. I was on the ground and I just dragged myself to the neighbour's house and Eimhear just followed me and was crying."
As well as losing a kidney and part of her liver, Brenda lost part of her bowl, gall bladder and spleen and suffered serious injuries to her right arm.
She was unable to attend her sister's funeral during which Bellaghy parish priest Fr Andy Dolan described the murdered teenager as a "gentle soul".
Both the O'Neill and McNally families have suffered tragedy in the past.
On the day Phelim McNally was born in 1988 his father - also Phelim - was shot dead by the UVF at Ballinderry, near Coagh, Co Tyrone.
The O'Neill family also suffered loss in 1987 when their Brenda and Lauren's aunt Geraldine and her husband Liam Casey died after their car plunged into the River Bann at Toome, Co Antrim leaving a young family behind.