Fate StayNight is owned by Type-Moon. I am not Type-Moon, and I'm not making any money whatsoever out of this.
If you can see any similarities between the characters and the events of this story and anything or anyone from real life, get your eyes checked, pal.
She had lived.
Thanks to some help from Tohsaka and some others in a few strategic places, they even had found a way to keep her alive indefinitely. Naturally, she had decided to stay at Shirou's.
Back then life seemed so bright and colorful, so hopeful and radiant, each day a new carnival. She could tell Oniichan was still upset at himself and more than a bit shaken over losing Saber, but she knew he would get over it. Eventually. After all, he had her, wasn't that right? And he always made sure of smiling at her, no matter how he was feeling. Ilyasviel just knew soon, very soon, all those smiles would become genuine.
She was half right. They enjoyed their time together, but soon it became evident things wouldn't turn out the way she had expected. He wouldn't forget, no matter what, no matter how much effort she put on making him forget. And it was frustrating, oh so frustrating, and more than once, she yelled on his face how stupid he was being, how he didn't have enough with her, with her company.
Shirou would only smile apologetically, with a smile that turned sadder every year, and humbly say, "I'm sorry, Ilya-chan."
He never stopped sitting on the porch, to look up at the crystal blue sky, as if he was waiting for... her. Not her, but her. Occasionally, he also would go away for a few weeks to make research, sometimes with Tohsaka, and would always return empty-handed, apologizing to Ilya and bringing her gorgeous gifts that never managed to make up for his absence.
Ilya was not growing up. She understood finding a way to fix that up was also another reason why he often went away, but she had no issues with being like she was, on itself. She felt perfectly okay being small and cute, but if Oniichan wanted her big and adult, she would go along with it. But she knew his intentions weren't making her his bride anyway. That only made it all the more painful; she felt like refusing his offers, and yet she couldn't.
"If I were a big woman like Tohsaka, would you-?"
"Ilya-chan, please, we have discussed this..."
She was no one's fool. She knew Shirou was a man, and a man with needs. From the tiniest things, she knew what he and Tohsaka used to do at times during those trips, when the need for company overwhelmed everything else. Not that Ilya was truly jealous, because she knew, in the end, Tohsaka had as little ultimate claim on Oniichan's heart as she had, or even less. They both were more or less just as behind compared to Saber.
But still, Tohsaka at least could have his body, even if every once in a while. Eventually even a child, that time when Ilya guessed something didn't go with the plan. That was a sharp blow, but she adapted. She even came to actually like the boy whenever his mother brought him, although she wouldn't admit it to anyone.
By then, they had moved far into the countryside, because it had been two decades now, and Ilya still looked the same. Still a virginal child, no matter how many times she snuggled into Oniichan's bed to sleep with him. Every time, every night, they only slept. Ilya never stopped wondering what was wrong with her, why she just didn't claim her deserved prize by force. The old Ilya would have done that.
But she couldn't ever do anything but resting in his embrace, uneasily.
She had grown used to living in modesty after being surrounded by luxury, but she never got used to being only the little sister. It was cruel to her, couldn't he see it? Of course he could, yes, but in his own way, he was as powerless to remedy that as she was.
Ilya still endured.
And Oniichan grew old and ill, and gray, and Ilya still was the same, still a woman with needs in a body that never could meet them. And he never stopped sitting on the porch longing for the long gone woman, and she never stopped sitting at his side, holding hands.
"When you're gone, what will I do? Where will I go?"
"Rin has agreed to take you in," he said. "It'll be nice, you'll see. You'll have a whole lot of books at your avail, and a huge fancy house again, and you'll get to see Kiritsugu-kun as often as you want. It'll be far better than being here."
No, it wouldn't be, because he wouldn't be there. But as always, she endured, holding the harsh, unspoken truths of protest in her chest, for his sake. She only asked, "Where will you go?"
"I don't know, Ilya-chan. I don't know."
But she knew, in his heart, he was adding, "Hopefully, with her."
She knew him better than himself at that point.
Squeezing the bigger hand of her brother, Ilya looked at the sky with him.