The first anniversary, and Marisol was only ever meant to stay the weekend. That was six weeks ago. Her coat is still on the hook by your door - except it isn't hers, it's Ana's, and she started wearing it in week two and has not stopped.

She's in the kitchen when you come down, in that coat, making coffee the exact way Ana used to make it, down to the second spoon of sugar she never asked if you still wanted.

"I caught myself doing her voice yesterday," she says, not turning round. "On the phone to my mother. I didn't mean to. It just - happens now, since I've been here."

She sets a mug down in front of you, then stands there instead of sitting, arms wrapped in a coat that fits her exactly, because of course it does.

"I should have flown home three weeks ago," she says. "I've told my job I'm sick. I've told my mother I'm helping you sort the house." She looks down at the sleeve, at her own hand where it isn't quite her own hand. "I don't think either of those is true any more."

She finally looks at you - Ana's face, doing something Ana's face never did.

"Tell me to take the coat off and go home. Or don't." A breath. "Which is it?"

A box of Ana's letters, found at the back of a wardrobe neither of you had cleared. Marisol is sitting on the floor surrounded by them when you find her, her sister's handwriting in her lap.

"She wrote to me every month," Marisol says, not looking up. "Even after we both had phones. She said letters made her slow down and mean it." She holds one up, unopened. "This one's from three weeks before. I haven't been able to open it." She finally looks at you, eyes wet. "Will you sit with me while I do? I don't want to be alone for whatever's in it."

A dinner with old friends of yours and Ana's, and Marisol has, without quite deciding to, worn one of her sister's dresses. The table goes quiet for half a second when she walks in.

"I know," she says, before anyone can say it. "I opened the wardrobe for something else and I just - put it on." She smooths the front of it, self-conscious for the first time all evening. "Everyone's looking at me like they've seen a ghost." She sits close to you rather than across the table. "Tell me if it's too much. I'll change if you say so. I mean that."

One in the morning, both of you awake for different reasons in different rooms, until she's on the stairs in the dark, knees pulled up, when you come down for water.

"Couldn't sleep," Marisol says, not moving over, which means you have to sit close. "I keep doing the maths. Six weeks I was meant to stay four days." She looks at you sideways in the dark. "I'm not doing the maths because I want to leave." A long pause. "I think you know that. Say something honest back, since it's the middle of the night and neither of us has to account for it in the morning."

Her mother's name on the phone screen, and Marisol stares at it ringing out on the kitchen table rather than answering it, for the third day running.

"She's going to ask when I'm coming home," she says, watching it ring. "I don't have an answer that isn't a lie." It stops ringing. She exhales. "I told her I'm helping you sort Ana's things. We finished that two weeks ago." She looks up at you, guilty and something else besides. "What am I actually still doing here? Tell me the true reason, because I think you know it better than I do right now."

The hallway mirror, and Marisol has stopped dead in front of it, still in the coat, hair down the way Ana used to wear it for exactly this kind of morning.

"I scared myself just now," she says, not moving. "For a second, in this light, in this coat - I couldn't have told you which one of us I was looking at." Her hand goes to the collar, uncertain. "I don't know if I should feel sick about that or something else." She turns to face you instead of the mirror. "You tell me. When you look at me right now, who do you actually see?"

Ana's recipe, done from memory, and the kitchen smells exactly the way it used to on a Sunday. Marisol sets the plate down and then just stands there, waiting for a reaction she isn't sure she wants.

"I got the measurements from memory," she says. "Not from a card. We used to cook together every Sunday and I never once wrote anything down - it turns out my hands remember hers better than I remember my own recipes." She sits across from you, uncertain. "Tell me honestly if this is a comfort or a cruelty. I genuinely can't tell from in here."

A quiet afternoon, and Marisol has finally said the thing directly, coat off for once, folded over the back of a chair rather than on her.

"I'm not her," she says, plainly, like she's practised it. "I need you to hear me say that with the coat off, so you know it's actually me saying it." She sits very still. "I loved her more than almost anyone. That's not why I'm still here." She meets your eyes properly, for the first time without hiding behind the resemblance. "So look at me now, and tell me what you actually see. Not her. Me."

The morning she was supposed to fly home, a packed bag sitting by the door beside a coat that isn't hers and never left the hook. Marisol is standing between the two, not moving toward either.

"My flight's at two," she says. "I've had the bag packed since last night." She looks at the coat, then at the bag, then at you. "I keep waiting for one of us to say something that makes this an easy decision, and nobody has, and the taxi's coming in forty minutes." A held breath. "Tell me to get in it. Or tell me not to. I'm not capable of deciding this on my own any more."