Livvy has been brought up by her mother and her aunt. The three have lived as a family since she was born – and it has worked beautifully. Now they are campaigning to get alternative family units like theirs recognised in law

Catherine and Ginda Utley have entirely shared the raising of Livvy; but they are not a gay couple and that has caused them somewhat of a problem. In fact, they are sisters, siblings who have stuck together through thick and thin. When Catherine unexpectedly found herself pregnant in 1993 and knew she would be raising her child alone, Ginda didn’t hesitate to offer to step in and be by her side.

Two decades on they are as secure and stable a family unit as you could hope to find, and now Livvy has graduated they are all back together again, in the terraced house in Battersea, south London that they bought together when Livvy was a toddler.

But it’s the house that symbolises their difficulties because two sisters who live together receive no recognition in law as a family or couple when it comes to inheritance tax. “What that means is that if Ginda or I died, the surviving one of us would be forced to sell the house, uprooting Livvy from the home that has been the bedrock of her life, at a time of enormous grief,” explains Catherine.

The issue would be the tax bill: because as sisters the women are unable to transfer property or capital to one another in the way married couples or gay couples in civil partnerships can do. If one of them died, the sister who survived would be likely to be left with a tax bill of at least £100,000, and possibly a good deal more.

“It’s because house prices have gone up so much since we bought it, and of course we don’t expect anyone to feel sorry for us because of that,” says Catherine. “But at a time when the definition of what a family is is being broadened out, we feel aggrieved that our own very real family unit isn’t recognised – and by a Conservative government too, whom you might have thought would applaud us for providing exactly the sort of secure family unit they are always waxing lyrical about.”

What the Utley sisters want is a legal change to allow long-term cohabiting siblings the same rights as those enjoyed by civil partners, either by being allowed access to civil partnerships or else by addressing the rights of cohabiting siblings in separate legislation.

They are currently engaged on a round of meetings with MPs from all parties and say there’s strong support for change, with a private member’s bill in the offing. In 2008, a similar campaign waged by two elderly siblings, Joyce and Sybil Burden, then 90 and 82 respectively, ended in failure when the European court of human rights ruled against them. But, say the Utleys, since then much has changed, particularly a widening of the definition of what constitutes a family, and the advent of same-sex marriage, which in turn changes the status of civil partnerships.

Both in their 50s, Catherine – who has spent her career working in the BBC World Service – and Ginda, who is a secretary, have had a life on the edge of politics: their father, the journalist TE Utley, was Margaret Thatcher’s speechwriter and after he died she described him as “the greatest Conservative thinker of his generation”.

In the past – especially after the loss of so many men during the first world war – sisters who lived together over a lifetime were relatively common.

“Of course there are fewer of us now, but as families change and fewer children are raised in a traditional two-parents set-up, extended families are becoming much more important – and it’s important to give us recognition,” says Catherine.

The irony for Catherine is that if she and Ginda were simply best friends raising a child together, they could have a civil partnership – there is no requirement for a sexual relationship and the reason blood relatives are excluded is simply because the 2004 legislation that ushered in the civil partnership was designed to make it as similar as possible to marriage, from which close blood relatives are barred.

At the heart of the issue, says Catherine, is the question of what constitutes a family. “You know we’re a family,” she says. “You’ve seen how we operate.”

Indeed I do: my daughter Ellie and Livvy have been close friends since nursery school. Like many parents at the nursery, I was initially confused by the rather odd “mother” of my child’s friend. She seemed to undergo a slight but mysterious and perceptible personality change through the course of the working day: at drop off she tended to be chattier and more giggly; and sometimes, when I took up the thread of a conversation at 3pm that we’d been enthusiastically discussing at 9am, she seemed vague and hesitant about it.

Only later did I realise that this was because Livvy’s “mum” was in fact two people: her mother and her aunt. The sisters are exceptionally alike and often mistaken for twins. “Livvy went right through primary school with some parents never realising we weren’t one person,” says Catherine.

It has always been tricky for Livvy to explain her family’s unusual set-up to her friends.

“When I meet people for the first time I’ll bring up my aunt and I realise calling her that doesn’t quite explain the relationship. So I tend to collate my mum and my aunt into one person.

“Calling Ginda my aunt doesn’t begin to do justice to the role she has played in my life. I think the nearest I came to feeling it was understood was at primary school: my friends there used to say, ‘Livvy has two mums. That one there is her second mum.’ I think children are very accepting of things, and they ‘got’ our family in a way not everyone has. The truth is that my mum and my auntie are totally interchangeable for me – I couldn’t say I felt closer to one than the other. Ginda is completely a second parent to me.”

The hero of the story is undoubtedly Ginda, and Catherine’s tribute to the older sister who has been her rock is moving. “I remember the day I discovered I was pregnant; I was white with fear, I didn’t know what was going to happen or how I’d cope. Ginda’s immediate reaction was: I won’t desert you. She just moved in. She did most of the childcare. People used to say, how lovely to have a niece – you can always hand her back, can’t you? But Ginda couldn’t hand her back – she was usually the one in sole charge.”

Catherine believes that, ideally, it’s best for a child to be raised by a mother and father. “But we don’t live in a perfect world, and for us this arrangement has made all the difference. I can’t imagine how it must be to raise a small child on your own – they are hard work and not always great company. When Livvy was tiny, hearing Ginda put her key in the door was always such a great moment.”