It was in those beautiful moments when everything had settled down and the house was quiet and it was just her and the baby that she allowed herself to dream of becoming a mother.

"It was my way of 'mothering'," she said. "It was a close as I would get."

That was only part of a very long and very emotional journey that ultimately ended with the birth of her and her husband Garreth's daughter, Ruby, now nine-months-old.

Mrs Wigg, 30, could not have children because a series of blood clotting disorders made carrying a baby too risky. Her baby was unlikely to survive in the womb beyond 20 weeks; she would be susceptible to a stroke and other problems.

Ruby became possible when Mrs Wigg's sister, Emma Fowler, of Bonython, offered to be her surrogate, way back in late 2013.

Mrs Wigg and her husband, both teachers, said they would never have asked Emma to be a surrogate and were blown away by her offer.

"We were so shocked. We really didn't know what to say. We were so thankful and excited but we really didn't know where to start," she said.

That lack of information is not least the result of a ban on advertising for surrogacy in the ACT, as elsewhere in Australia. The families eventually found their way to the Canberra Fertility Centre which does facilitate unpaid surrogacy.

Kate and Garreth provided the egg and sperm for the embryo that was transferred to Mrs Fowler after a very long, emotional and involved approvals process. Mrs Fowler says she had no hesitation handing Ruby over to her parents when she was born in May last year.

"From the very beginning, I just had it set in my mind, 'This baby is not mine, it's biologically theirs and I'm literally just the oven, I'm just keeping it warm and safe until those nine months are up'," she said.

"There was never any doubt. I never crossed that line. If someone said, 'Oh, congratulations you're having a baby', I always said, 'Thank you, I'm actually carrying for my sister'. I never allowed there to be a grey area.

"When it actually came to the birth, there was not even a hesitation. It was just amazing she was here and they were a family. When it came to the birth, I was very protective of Ruby and really loved her but I didn't feel she was mine. She wasn't mine to begin with."

Unpaid or altruistic surrogacy – such as that in which the two sisters engaged – is legal in the ACT. Commercial surrogacy is illegal. The ACT also bans its citizens from going overseas to use a commercial surrogate.

A federal parliamentary inquiry is now investigating the laws and regulations around surrogacy and is due to report in June. Close to 100 submissions have been received, many calling for surrogacy laws across Australia to be made more uniform and for paid surrogacy to be legalised. Others reject the prospect of any surrogacy being allowed in Australia. Concerns have been raised about exploitation of women used as surrogates overseas and the welfare of children born via surrogates in other countries.

Submissions have suggested as few as 14 babies a year were born in Australia through altruistic surrogacy compared to as many as 270 babies born through the use of overseas paid surrogates.

The Canberra Fertility Centre is advocating for the introduction of regulated, paid surrogacy in Australia. The centre's medical director Dr Martyn Stafford-Bell has suggested fertility centres would act as the go-between for surrogates and families, with the same fee offered across Australia to avoid competition.

He believed paid surrogacy would go through the same journey towards public acceptance as IVF.

"I simply see surrogacy as just another branch of assisted reproductive technology," Dr Stafford-Bell said. "It's just totally logical that we should do it."

Mrs Fowler said she "had it in the back of her mind" for years to offer to be her sister's surrogate, even before she had her own children.

"After I had my children and the struggle became more real for Kate, I just thought, 'We were done having children, why not? Why can't we help her?'," she said.

Her husband Cameron and children Brayden, seven, and Isabelle, five, were on board "100 per cent".

"I told the kids, 'Aunty Kate's tummy is broken and she wants to have a family like ours and we're going to help her'. They were just, 'Yep'. They were excited and proud, they were happy. And they understood what was going to happen. They just got it.

"We do have a special bond with Ruby and they really love her but she is their cousin."

The women had to agree to a series of processes before they started including counselling, police checks, medical checks, legal advice and approval from the John James Hospital Ethics Committee.

They were subject to a three-month cooling off period. After the first egg pick-up, the embryos were frozen for six months before they could be transferred.

Mrs Fowler was not paid but all her medical costs were met by her sister and brother-in-law. They estimate it cost them more than $80,000. While couples using IVF received the Medicare rebate, couples using a surrogate did not, something Mrs Wigg said should change, particularly for those couples using a surrogate for medical reasons.

She also found some resistance, even within the community, to accepting her as the mother of Ruby. But their obstetrician Dr David O'Rourke was their saving grace.

"He was one of very few specialists and people that really got it," Mrs Wigg said. "He always made Garreth and I feel like the parents and treated us like any expecting parents. When we would be in the waiting room he would call out 'Wiggs' rather than call out Emma's name. It was the first time we were acknowledged by an outsider as the parents."

The Wiggs' obtained a parenting order from the Supreme Court which saw the Fowlers relinquish any parenting rights over Ruby to them. However, the Fowlers are still listed as Ruby's parents on her birth certificate. And her name is still on Mrs Fowler's Medicare card, not Mrs Wigg's.

The sisters are the daughters of Ruth and Peter "Butcher" Lindbeck, who supported them all the way.

"We couldn't have gone through the journey without the love and support of family, friends and colleagues," Mrs Wigg said.

"My parents are amazing and if we could be even half as good parents to Ruby as they are to us we would be very happy."

And, like when she was a night nanny, it is in the dark, in the middle of the night, when she is feeding Ruby that Mrs Wigg feels that rush of being a mother.

"I always think about my sister and her husband with grateful tears running down my face. I hold Ruby tight and kiss her a zillion times. We really feel like the luckiest people on earth."