Katie's sister Anna was 28 weeks pregnant when her family got a sickening phone call.

“Every time I set eyes upon a squishy, beaming little infant, I am instantaneously possessed by a primal urge to procreate. A deep, aching longing sets in such that it’s all I can do not to grab the thing and run.”

Five years have passed since Melbourne writer Katie Horneshaw penned those words on her blog.

That ache, that longing has eased now that she has three-year-old Ollie, a little boy who came into her life in a way no one could have predicted.

Though Ollie calls the 32-year-old Mummy, though he calls her partner Dad, though he is their son in the eyes of their family and the law, biologically speaking, he is their nephew.

Katie’s sister Anna gave birth to him just six months after being jailed for murdering her elderly flatmate during an argument in 2015.

(For more about Anna’s story, read this extract of her mother Mary K. Pershall’s book, and listen to her interview on No Filter below.)

As soon as the family learned of Anna’s pregnancy, there were concerns. She suffered with mental health issues, had struggled with addiction to ice and had displayed violence toward her own father.

Even in those early weeks, Katie knew what it meant – she was going to become a mother, just not in the way she and her partner had hoped. The thought of raising a potentially compromised baby frightened her. She even urged her parents to convince Anna to terminate the pregnancy.

But once Anna was imprisoned and the time for that had passed, Katie changed her mind. She knew there’s was no Plan B, not even external adoption.

“Anna was already having to go through so much, she was already having to face the prospect of her child being removed from her at birth,” she told Mamamia‘s No Filter podcast. “The last thing that she needed to go through was then the prospect of having him adopted out by somebody she didn’t even know and potentially getting very little contact with him.”

Three days before Ollie was born, it was confirmed Katie would become his guardian. Three days was all she had to prepare to become a mum.

The birth, she said, was "a mixture of joy and pain". Joy at this wide-eyed, bright, healthy little boy. Pain knowing her sister barely got to hold him before he was whisked away to special care and she back to prison.

"Even though she'd done such a terrible thing it's still horrible to see a mother separated from a newborn baby," Katie said.

Katie now takes Ollie to see Anna at Dame Phyllis Frost maximum security women’s prison every Sunday. He calls it "the big house", he calls Anna 'Mum'.

But to him she has an aunty role. Something Anna has adapted to better than Katie ever thought she could.

"She's just so glad that he's with family and [that] she sees him so often, because she sees other women in that prison whose kids are in state care," Katie said. "They don't see their children for months, sometimes years, on end. They all tell her how lucky she is, which is good for her to hear."

Ollie will be 13 by the time Anna is released. A decade to process the situation, to prepare for this woman to take a bigger role in his life.

"I imagine that their relationship will just continue to be fairly similar to what it's been," Katie said. "If she can provide a safe place for him, I'm more than happy for him to go visit her and but I don't think that she has any illusions of thinking that she'd ever take him on full-time."

As for whether Katie will have a child of her own...

"[My partner and I] were definitely having a bit of trouble," she said. "So to be honest, maybe not. Because now I'm concentrating on my career and I'm kind of just happy with Ollie at the moment. So the way I'm looking at it is if it happens in the future it happens, but if it doesn't it doesn't. And I think I'm totally happy either way."

To hear Katie's chat with Mia Freedman, listen to No Filter below or via your favourite podcast app.