Shirou-Miyuverse is an alternate timeline where the Holy Grail War operates under COMPLETELY different rules, and of the 'three founding families' only the Makiri's/Matou's exist. Instead it operates via 'class cards' where one takes on the powers of a Heroic Spirit and they fight it out.
There exists a family, in Japan, that masters a form of Magic that grants wishes, similar to the end goal of a HGW. This wish-granting power can only be used by children of the family, and goes away as they get older. For generations they kept their children locked away, and merely used the power to wish for their children to have happy and healthy childhoods/lives.
One of the families behind the HGW want to 'save the world' and 'end all suffering', the reason they helped create it. They find out about Miyu's family. An explosion happens, Kerry and a young Shirou find Miyu as the only survivor.
Kerry dies, having found out about Miyu's power and tells Shirou to sacrifice Miyu to use her to save the world.
Shirou decides his love for his new sister was more important than his dream of saving everyone. They live happily for a few years, then Shirou(weighed down by guilt) brings Miyu to her old home to explain everything.
The last member of that foudning family shows up, kidnaps Miyu, and has someone working for them kick the shit out of Shirou while using the Gilgamesh Class Card.
Months pass, Shirou training and trying desperately to find a way to get his sister back(This is dead eyed shirou from the video). Suddenly Sakura, who confesses her love for him and begs him to run away with her before the Grail War begins. He refuses, so she offers up her Class Card, the Archer card.
Then Shinji shows up and kills her, while revealing it was a fake card the whole time because they knew she'd betray them. He uses the Assassin card. After she dies Shirou uses the fake Archer card to bond with a version of himself, a fake card for a fake heroic spirit after all.pt2