Natalie, 32, had been watching the World Cup final with her family on the day she was murdered on 18 December 2022.
A post-mortem examination found she endured a “prolonged assault” that included multiple stab wounds, strangulation and several heavy blows to the head.
Her family were watching the World Cup again on Tuesday when they were informed that McCullagh was appealing his sentence.
“It just speaks to his character completely,” Declan said.
“That he would do it just speaks to his hideous nature. Then his behaviour since that he’d put us all through a horrific trial despite all the overwhelming evidence.
“And at the end he now expects to have an appeal and get the sentence reduced.
“If they’re going to look at the sentence again, the only way the sentence should be going is up,” Declan added.
McCullagh had returned to Natalie’s’ house the day after the murder to pretend to he had discovered her body and made a distressed 999 call.
He was arrested at the scene but later released because he told police he had been live-streaming himself playing video games on YouTube at the time of the murder.
After the murder he presented himself as a grieving boyfriend and attended her wake and vigil.
At his trial the court heard that he secretly recorded conversations at her family home by deliberately leaving his mobile phone behind.
Prosecutors said he did so to find out whether her family suspected him or had discussed the police investigation.
