Rider and Shirou
Rider was listening to Vanguard explain the nature of their world to Shirou when, suddenly, she noticed Sakura run down the stairs in a panic, heading for the basement.
"Rider, what is happening?" asked Vanguard.
“I don't know, but Sakura looks worried. I'm going to go check it out, she could be in danger”, she responded.
As she did so, she noticed Rin charging after her sister. Her magical signature showed that she was reinforced and ready for action. Rider charged after her, heading towards the basement, hoping that Sakura would be OK.
Meanwhile, Shirou continued to comfort his wife, kissing her and rubbing her back gently, trying to soothe her. Noticing that she had gone limp, he panicked for an instant, before noticing that she was breathing normally, and seemed to just be asleep.
Well, she has had a busy day, I suppose, he thought, continuing to comfort her.
I guess I should find somewhere for her to sleep comfortably, he thought, giving her one last kiss before preparing to shift her off him so he could carry her up and out of wherever it was that they were.
Before he could do so, however, he heard a “thump” on the carpet. Reinforcing his eyes, he looked up, but before his eyes could acclimatise to the darkness enough to see anything more than a blur, he heard a cough, followed by another familiar voice coming from the woman.
"Well, I'm glad I rushed out of the shower to find that Emiya-kun really does like younger girls”, she said.
Eh, what? She's only a couple of years younger than me, he thought, confused.
Before he could say anything, though, Sakura suddenly jumped up and pushed him away.
W-w-w-w-what!” she exclaimed.
Shirou was now throughly confused.
“Sakura, what's wrong? And, Rin, Sakura is only two years younger than me, and she's my wife”, he said, before his pain-addled brain did a double-take.
"Wait, Rin, why are you even here? What is going on?" he continued, thoroughly confused at the situation.
Rider, however, knew exactly what was happening. She wasn't sure if this was the Shirou from her world, but she certainly couldn't detect any differences. Either way, in the pitch black of the basement, he had obviously mistaken the much younger Sakura for his wife, and had kissed and comforted her accordingly.
“Shirou, that's not your wife”, Rider said with a laugh. “She is far younger than your Sakura, and has her own Shirou."
Shirou looked up at Sakura again. With his eyes now somewhat acclimatised to the darkness, he was able to make out some features, and he did notice some slight differences, although it was difficult to tell in this light. He also noticed that Rin seemed to be wearing only a towel.