Her two are privately schooled, while her partner's daughter goes to a state school - and, when they moved in together, he said he was happy with this.
Now, though, the daughter is demanding that she should go to the same fee-paying school as the other children - and that, if not, they should be removed from their school.
"I said to [my partner] 'so what did you say??!' And he said 'oh I just told her we'll look into it', she writes.
The dilemma is complicated by the fact that the partner can't possibly pay the third set of school fees himself, as he's on a very low income.
"I'm feeling like a complete mug now. He's already moving into a house which will be paid for mostly by me as his wage is so low and he has no savings so I've paid the deposit too," the woman writes.
"Am I being unreasonable to tell me I'm not paying for her schooling? And I'm certainly not taking my kids out of their school?!"
Opinion is divided amongst other posters on the site.
"If you're all moving in together surely you're going to be a family. If not what's the point?" asks one.
Another says "I do think you should aim for 'fairness' as much as possible. Having one sibling have a very different upbringing to the other two could easily end in a 'Cinderella' type situation if things aren't carefully handled.
Others, though, suggest that the girl's schooling is for her own parents to sort out.
"It would be different if you chose to put yours in private school once you were living together but why should your children be removed now because his daughter doesn't like it? Her education is the responsibility of her parents! Does your children's father contribute to their school costs?," one poster says.
Another points out that things could get more complicated down the line: "If you do decide to pay for her schooling then he needs a plan for what he'll do if you split up. He can't expect her to yo-yo in and out of schools," she writes.
In law, of course, the child's parents remain responsible for financing her, and the Mumsnet poster has no obligation to pay a penny even if she and her new partner eventually marry.
In practice, though, there are clearly potential problems with treating one child differently from the others.
Couples should talk things through and consider having a cohabitation agreement drawn up by a solicitor before taking the plunge.
What do you think about this situation? Let us know in the comments below!
Your best childhood holidays
Your best childhood holidays
"My mum used to take three children, a dog and a huge family tent in a Triumph Herald. We camped for the whole of the summer holidays - it was the best time ever as kids." Chevaune Stanley
"It's where I lived as a child so life was one long holiday. Have taken my husband there a couple of times too." Ellie Long
"Tenby has four superb beaches. I remember playing in the dunes at the back of South Beach, building sandcastles and visiting the café on the Castle Beach, catching a boat to Caldey Island from the Harbour Beach, and paddling in the sea at the North Beach. Oh, and enjoying ice cream at Fecci's Ice Cream Parlour! It seemed like a magical sunny place when I was a child." Bel Merriman
"Scrambling around Dartmoor National Park, picking heather and pretending I was in Wuthering Heights... my grasp of geography has now improved!" Emma Sleight
"We went every year with my parents and grandparents - such great memories. Very clean beach, lovely locals and fantastic hotels." Leanne Nash
"Just provided so much laughter and fun for us as a family." Marie B Yates
"I have wonderful memories of sunny summer days camping with my parents. It's amazing sandy beaches that went out for miles to the sea, surrounded by the awesome mountains and not forgetting a ride on the steam train. I'm going again very soon!" Lynn Evans
"First ever family holiday abroad." Sue Fenney
"We used to go their as a family, sit on the harbour and the seagulls would come down and pinch our chips from our paper. Then we would go and have a proper knickerbocker glory with fresh fruit and ice cream and lashings of cream. Mmmmmm those were the days." Paula Smith
"Was still a commercial port with a small timber trade and fishing fleet. Crabbing and fishing from the quayside, hours spent on the beach. And being allowed out alone on the harbour and cliffs at ages 10 to 13 was a world away from a London childhood. A timeless seaside holiday!" Janet Govey
"Brighton holds lovely childhood memories for me, going with my mum, dad, brother, grandad and uncle." Chris M Williams
"Mum would walk me and my eight brothers and sisters there every day during the summer holidays." Christine I'anson
"We always came to Wales - Rhos-on-Sea and Llandudno. I now live there." Hilary C Williams
"...because it had everything a nine year old could ask for." Laura Hyland
"I always went to Worthing which is not the most exciting place in the world, but it was where my granny lived! And I loved to go shrimping and the beach was very safe. I would spend the whole day on the beach building sandcastles and paddling in the sea." Sara Hill
"The beach was amazing and my dad and I used to spend hours watching the surfers." Brogan Driscoll
"We couldn't afford to go on holiday as children. Mum and Dad had nine of us but as we live in Poole, Dorset we used to walk to Sandbanks Beach every day through the summer holidays. It took at least a couple of hours walking but oh so worth it." Christine I'anson
"25 years of family holiday memories and meeting a lovely family that helped make those memories." Paula Cuff
"Just stunning views and good beaches." P and J Hotel
"Great beach, stunning views, large swimming pools and free food in the resort." Adnan Kekhia
"Could never afford to go abroad." Kate Mo Edwards
"We went to Torremolinos on the Costa del Sol for six years running and I have so many fab memories! We always stayed in the Sol Principe Hotel!" Paula Blair
"Beautiful place and near Blackpool Sands, fab sandy beach, seagulls crying and seafood, sandcastles and Devon hospitality! Loved it!" Janelle Riley
"Tenby in West Wales was my idea of Heaven! Sheltered sandy beaches, a bustling harbour and boats crossing the choppy channel to Caldey Island. We would collect cockles for a special treat. Very Enid Blyton and idyllic!" Anne Lee
"Tin mines and Mevagissey. Beautiful scenery." Rachel Lee
"Now it may not be particularly attractive but as a family in the late 60s early 70s my parents wanted us to have an affordable holiday together. We used public transport to get there and stayed in a caravan - daily treks to the tap to fill up the water containers, bucket for the loo during the night. Poor old Dad getting the job of emptying it! We loved it." Sandy Jimson
" A caravan park. Wonderful memories with the grandparents." Claire Sharp
"My favourite was Portugal as I used to love the specatular waterpark." Corinne Faulkner
"Broadstairs, Kent was our holiday for years. Sandy, shelving beach with donkeys, ice-creams, beach huts and a great bed and breakfast where the same people came back year after year. Lots of friendships made, sandcastles built and 2ps spent in the amusements!" Rebecca Walsh
"We went every year and rented a beach hut - it was fabulous to me because my Mum and Dad could even boil their kettle in there to make a cup of tea. Also I was fascinated by Peasholm Park and the Naval Battles that took place every summer on the boating lake - memories!" Jane Knape
"I used to go to Teignmouth in South Devon. I always remember happy family holidays and lovely weather (what happened to that!). I went back there two years ago with my now elderly parents, my husband and our eight-year-old daughter. It hasn't really changed and is still very traditional and not overly commercialised. It was nice for our daughter to see - she even went on the same Ladybird ride on the pier that I went on and remembered so well. It was like going back in time." Caroline McCabe