I gasp. Emma clutches my arm so tightly it hurts.

Ruby stops talking, refolds the letter and slots it back in her pocket. Her voice was shaking near the end of the letter, yet she remains dry-eyed, her face colourless and unreadable under the security lighting.

I shift my gaze to look at Dad. He’s staring at Ruby, shaking his head, tears streaming down his face. I can hardly believe what Ruby has just read us. But it must be true to elicit such a reaction from Dad.

‘In case you didn’t “get it”,’ Ruby says, making air quotes, ‘Tony here is my biological father. Which means…’

‘… we’re your sisters,’ Emma finishes.

‘Surprise!’ Ruby says without enthusiasm. ‘I did some family history research and tracked him down on Facebook. I found you two first. Only I wasn’t sure you were related as you weren’t Facebook friends. But you both looked similar to each other, and to me. And the name fit. And you were both the daughters of Tony Beresford, who was originally from Bristol, but who moved with his family to Malmesbury just before I was born.

‘So, from having no family at all, I found out I had this whole other family. Only, you didn’t want me. You would rather I was dead and buried. You would rather I had never existed at all.’

‘That’s not true!’ I cry. ‘If we’d known you existed we would’ve—’

‘You would’ve what!’ she spits, putting her hands on her hips. ‘You would have welcomed me with open arms? I don’t think so. You’re both as bad as he is. You two had each other. You had everything. But you let some stupid fuckwit get between you. How can you let an idiot boyfriend ruin your family bond? You’re sisters. But you ignored each another for years. Neither of you deserves to have a sister. You don’t deserve your family. And Daddy doesn’t deserve you, either.’

She begins pacing up and down by the side of the pool. If only she were on this side, I could charge and push her in, buy us some time to free Dad and get out of here. Call the police.

‘So, yeah, I sent you the letters,’ she continues. ‘I thought, if a letter is good enough for dear old Tony Beresford to send to my mum, then letters will be good enough for his daughters. Letters, and a few other little additions to spice things up,’ she says with a smirk.

I shudder when I think back to my day in Bristol. Being pushed into the road. Seeing a flash of auburn hair – hair exactly like Emma’s. But it wasn’t Emma. It was Ruby. ‘You pushed me!’ I cry.

‘I did. I pushed you both. And you fell, and you cried and you whined and you were scared. And I was glad. Because I had years of being scared and pushed around. And Mum couldn’t protect me all the time. I wanted you to have a taste of what it was like. Of what your dad let me and Mum live through. He could have helped us out. He could have checked up to see how we were doing. But instead, he wrote his little letter and he ran away and left us to it.’ She stops pacing and turns to look at my dad. ‘Out of sight, out of mind, hey, Tones?

‘I was just a dirty little secret. Something to be ashamed of, while you two were Daddy’s pride and joy. His two precious little girls. Yeah, well, thanks a lot, Dad. Thanks for nothing.’

I don’t even know what to say to her revelation. Part of me can understand her anger. But the things she did… they’re not the actions of a sane person. And I have no idea what she intends to do right now. Is she capable of hurting Dad? Of carrying out her threat to try to drown him?

‘You know what gave me the most satisfaction out of all of this?’ she says. ‘Hurling the ashtray through Georgio’s window. That ashtray was the only thing of Dad’s that Mum owned. I treasured that piece of marble all my life. When I was a kid I used to stroke it. And I would think, my dad may be dead, but he touched this same piece of marble that I’m touching right now. And I thought by doing that I would be closer to him in heaven. How pathetic is that! Especially as he wasn’t in heaven, he was a few miles up the road in Malmesbury. And instead of a little kid stroking a filthy ashtray, I could have been stroking his cheek. So, yeah, it felt good to smash that sucker through the shop window.’

Her story is heartbreaking, and terrifying. She is a wild, unpredictable, angry creature. She’s been wronged, of that there’s no doubt. But this has to stop now.

‘Ruby,’ I say carefully, ‘I’m truly sorry. I honestly had no idea. And I wish you had been part of our lives. Dad made a terrible, terrible mistake. But now that we know… now that we know we have a sister… surely this can be a good thing. We can get to know one another. You can be part of our family.’

She looks down at the ground for a moment and then snaps her head back up. ‘I’ve seen enough of your family to know that I don’t want any part of it. You’re both a couple of stuck-up bitches. Your mum’s a clueless snob, and your dad is spineless. So thanks, but no thanks. I think I’m best off on my own.’ Her face is flushed now and she wipes away a loose lock of hair with a hand that’s shaking violently.

Her words are harsh, but she’s angry. And maybe I would be angry, too, if I’d had her upbringing. But whatever happened in her life, it doesn’t justify what she’s done to us, and what she’s doing to Dad. Yet I can’t deny that I’m shocked by my father’s history. He cheated on Mum. He has another daughter! But right now, I can’t dwell on that. He’s still my dad – and I need to help him.

‘What about Ian?’ I ask, trying to change the subject a little, trying to buy some more time so I can figure out what to do. ‘I take it you used our neighbour to get to me?’

‘Yeah,’ she says disinterestedly. ‘He was a small price to pay to be able to keep an eye on you. And you were oblivious, Lizzy. You bought my ditzy act, behaving like you were so much better than me. Patronising me and Ian like we were a couple of idiots.’

I shake my head. That’s not how I remember it. I didn’t patronise them, did I?

Emma takes a couple of tiny steps forward, and I shuffle up next to her. I get the feeling she’s about to do something, but I’m nervous. If she makes a sudden move, Ruby could hurt Dad, and I’m not convinced we could save him in time.

‘How did you know we’d come here?’ Emma asks.

Ruby gives a dry laugh. ‘You two were so easy to manipulate. First, I sent you the “wrongly addressed” letters to get you together. Then, once Joe was out of the picture, I sent Emma the fake text message to get you over to Lizzy’s house. I’d already sowed the seed of the holiday home in your mind, Lizzy, so I was ninety per cent sure you’d go for it.’

‘Did you really win a minibreak here?’ I ask.

‘You’re joking, aren’t you?’ She laughs. ‘I clean the place, that’s all. They don’t give away posh breaks to the likes of me. But I’m friends with the receptionist, and she let it slip that this house had a cancellation, so I thought it would be perfect for what I was planning.’

So we’re trespassing. We’ve been enjoying this house without anyone’s permission. And now I’m wondering what it is that Ruby’s actually got planned. Did she bring us here purely for dramatic effect, or does she really mean to hurt Dad? I know what he did was very wrong. Was awful. And she has every right to be furious. But I can’t let her harm him.

Emma and I inch forward a little further.

‘Anyway,’ Ruby says, walking over to the grey concrete parasol base next to Dad, ‘we can talk all night about this crap, but that’s not why we’re here.’

I break out into a sweat. My whole body tenses. I get the feeling I’m going to have to dive into that pool any minute.

‘No,’ Ruby says. ‘We’re here to watch Daddy dear get what he deserves.’

Dad’s eyes widen and he begins to struggle, but it’s no good. He’s tied too tightly and he’s weighed down by the enormous, thick chain across his lap.

Ruby bends down and pushes the parasol base, which rumbles and slides into the pool with a heavy plop. Mine and Emma’s screams drown out the hideous, slithering splashes as the chain unravels, pulled into the pool at an alarming speed.

‘Dad!’ Emma cries out.

I stagger to the edge of the pool, willing myself to stay strong, to not panic. I suck in a huge breath, preparing to dive. Preparing to pull my father out as soon as he’s yanked into the water by the slithering chain. Emma is by my side.

Any second now…