When Shirou(Miyu's brother) wished on the Grail for his sister to live a happy life, and after fighting Angelica he somehow appears with his sister in Fate Zero Fuyuki a year before the 4th Holy Grail War.
Rules
Female Gilgamesh
No single pairing
No ShirouxOC
Pairing whatever you want but if possible add Miyu.
Also Miyu still has the same tastes in pajamas(their adorable).
Miyu and Shirou are Masters in the 4th War you can have whoever you want to be their Servant but none of the Zero Servants they stay with their canon Masters. Also if possible let Shirou summon a Saber face like Saver (Saber Lily), Avenger(Alter Saber, Mordred, Alter Saber Lancer), and Ruler(Jeanne D'Arc) etc.
OR you can completely ignore my rules and do your own thing.
Also they live in a house similar to Prisma Illya's house, Kind of obvious but hey.