Charlottes
Miyu is actually blood-related to Shirou due to the wish she granted few years back when she and Shirou gazed at the night sky. A small talk come up that reach the point about siblings. Miyu at that time was still a wish granter, an ability that she inherited from her deceased family. She made that wish as Shirou little sister come true. Her eyes once red with golden shine turned golden-brown when the wish was brought forth. When she granted the wish as Shirou little sis. The wish actually made Miyu as Shirou biological sister. So even in the grand scheme of the multiverse. Shirou and Miyu are definite blood-related. Which effectively made EMIYA as Miyu big brother too.
LOL! There is a fanfic called super adaptability. The mc had the Overpowered ability of superior adaption. Unfortunately, the mc is gay which reduces the joy of reading an op protagonist story. And fortunately the gayness only happens near the end of the fanfic where he meets a Greek God called zangreus from percy jackson universe.