I will make no attempts to justify the timeline of this, but it's roughly mid-LB and early DH. Ignoring, of course the 50 or so years in timing.
"I'm just so sick of being left behind," said Ginny one afternoon, perched unhappily on the window-seat, nose not quite pressed against the glass. She was watching the people she called muggles again, with a strange sense of painful fascination. She couldn't bear them because they were too different, but Susan struggled because they were all the same.
"So stop letting them," Susan said, giving her tea a hard stir. "Drink your tea; it'll get cold."
Ginny turned, scrunching her face up, and for a minute Susan couldn't breathe, choked with familiarity.
Yes, she realised suddenly, hearing the cup cling against its platter. I could make her into Lucy. She could, perhaps, dress her up in the skirts Lucy used to love, and twirl bright ribbons into her hair, but it would never be enough to fool her, and Ginny would only end up hating Susan for it.
Ginny, for all the fire in her hair and the sharp bitterness in her words, could never be quite bright enough to make up for the sister Susan had lost.
"My mother used to say that," she said quietly, but she did jump down carefully, feet barely touching the ground as she landed before she was off again, throwing herself into the seat across from Susan. She picked up her cup carelessly, the tea sloshing inside, and stared down into its depths as if expecting mermaids. "I don't even like tea," Ginny added finally, frowning slightly.
"There's no coffee," Susan informed her, taking a long sip. It was a new brand of tea, but she already doubted she would ever dare to buy it again. It tasted like Calormene tea; bitter and smoky like the one Susan had drank every morning for so many years in Narnia.
It reminded her too much of what she had lost, so maybe Susan would drink no other type again, torturing herself in a vain effort to feel at home again.
"We could go and buy some," Ginny said, making a move as if to get up, running off somewhere again, as unwilling to sit still as Lucy had been. "Anyway, it's not as easy as that."
Lucy had hated the taste of tea too, she remembered, and bit down harshly on the inside of her lip, because it didn't, couldn't matter anymore, because no amount of feeble memories would ever turn back time.
"It's raining," she said instead, because in the gaps in their conversation she could still hear the constant thud of rain across the window, dull and insistent.
"Well, you won't melt you know," her companion muttered, before taking a small sip and grimacing. Susan pushed the sugar across the table.
"You might prefer it that way," she offered, because Lucy had, once upon a time. "There was mail for you this morning."
"Yes," said Ginny, "I saw."
She reached over for a biscuit, hand hovering hesitantly over the platter for a minute before finally deciding. But Ginny didn't lift it up to her mouth, just held it gingerly between two fingers.
Her nails were bitten to the quick. Susan's handmaidens would have detested her, groaning over each tiny flaw the way Ed had over a diplomatic crisis.
Susan just waited, because she had learned patience a long time ago, and it had served her well afterwards, waiting for something that seemed like half a dream.
"No news of the war, not of Harry, anyway," Ginny garbled out, "only the usual platitudes about Aunt Muriel's curtails and Dad's questions about muggles." She spat out the last word as if a curse, sneaking a glance at the window. "He'd love it here."
"You should answer them," Susan said, hiding a smile. She had withstood insolent demands from myriads of dignitaries, and one girl's lonely attempts to take her anger and boredom out on her made for little provocation nowadays. "It might take your mind off things."
"You should answer yours, then, if you're so sure of that."
"Oh?" said Susan, taking another sip. "I'm sure I don't know quite what you mean."
Ginny scowled at her. "Of course you do. From an L. Pevensie. Relation?"
"In a way," Susan acknowledged, though Lucy wouldn't, not these days, too stuck in memories of places and events Susan had put long behind her, because longing after a world she had been shut out of wouldn't do her any good.
"Estranged husband?" Ginny suggested, dipping the biscuit in her untouched tea until it turned from a pale cream to a soggy brown. "Softens them," she said, faced with Susan's inquisitive look.
"And makes for less tea, I expect," said Susan, "No estranged suitors of any sort, especially not ones who'd write me letters."
"Very estranged then? Bitter like the tea?" Ginny said, taking a long sip, the expression on her face almost a caricature of distaste.
Susan gestured at the sugar container again. "It's hardly unavoidable, dear."
Ginny dumped several generous helpings of sugar into her tea, and stirred so hard her teaspoon clanged noisily against the edges. "Whatever did you do to him?"
"No," she said, sighing, but Ginny was as hard to change the subject with as her siblings had always been, especially about Narnia. Perhaps pretending the other girl was Lucy wouldn't be that hard after all. "My sister, actually."
Ginny frowned, one hard line across her forehead, and oh, that was Lucy all over again. "You have a sister?"
"Yes," said Susan, and didn't mention the brothers. "I did."
"You still do, if she's writing to you!"
"Lucy and I," she paused, because there was no way to make anyone but the other three understand, "don't really talk," she finished, all too aware of how feeble it sounded.
"I don't talk to my brother Percy," said Ginny, somewhat defensively. "But no one in the family does either, so it's quite different."
"Then you may consider me the Percy of the family, and let us not talk. It's really nothing to do with you, anyway."
It was the wrong thing to say: Ginny tilted up her chin, and looked straight back. "That's what they always say." She did some contortions with her face, and mocked, "It's Order business," she sang in an off-pitch falsetto. "You're far too young."
"That's not what I said!" snapped Susan, who had hated the same as a girl nearly as old as those saying it.
"You said to stop letting people say things like that to me. That's exactly what I'm doing!"
"Some things are private," said Susan with a deep sigh. "You might understand one day."
Ginny laughed, bitter and mad like letting go, "You're just as bad as them."
She made herself smile, taking a deep breath, but her hand shook as she placed her empty cup back onto its saucer, and it rang out on impact in the silence of the room. "It's not about age, but experience and understanding. And in this matter, you have neither."