But when the gloves are off, their sibling bond is a force. 26-year-old Jonas shelved his own promising boxing dream to work two jobs and fund his sister's career, and Stephanie couldn't be more grateful.
"He's everything to me. If you can compare a human being with God I can compare my brother with God - what he's doing for me is amazing," she said.And Jonas said his sister was paying him back with her determination and steadily growing success.
"There's no other word I can actually express the joy that I have for her," he said. "I'm very, very proud of her."The Mfongwots arrived in Australia in 2009, fleeing violence in Cameroon where Stephanie lived with her grandmother.
"Seeing people dying so that wasn't very safe for me and scary," she said.Australia provided safe haven and specialist treatment for partial deafness diagnosed at birth. In Cameroon there was no treatment but Stephanie now wears hearing aids and the condition is improving.
"So what Australia has done for me, I want to give it back in a different way. I just want to do something that some people haven't done like fighting with a hearing aid," she said.Stephanie is already the reigning Australian amateur champion in the 69-kilogram division and has her sights set on an Olympic, then professional, career.
"If you can compare a human being with God I can compare my brother with God - what he's doing for me is amazing."Trainer Steve Kerr said she could do it.
"I've been in the game a long time and I haven't seen anyone move on their feet like her - men or women," he said.But he said it was Stephanie's determination that would get her there. She travels almost two hours by train, tram then bus to be trained by Kerr, who she now regards as family.
"The relationship I have with him is like daughter and father. I don't call him 'coach' anymore, I'm call him father."
And according to Kerr, the feeling is mutual.
"She just lights up the gym with her smile and everyone loves her," he said.With the support of her brother and trainer, Stephanie said she will give the challenging fight "game" her very best shot.
"I want to do something better to encourage people that have similar problem like me," she said. "It doesn't matter the situation you in, you can still do something as long as you love it".