[Ithekro]

I don't know anything past 2wei chapter 17 either.

Spoiler for Who is Miyu?:

Though it is sort of telling that Miyu's not a native to the world...and has the same birthday as Illya....and Kero. And that she thinks of Shirou was her brother. She sort of appeared when Sapphire needed a new Master. No clue about before that is really given. Irisviel seems to not call her a daughter, and if they split her from Illya, she'd know about it. She's not the Holy Grail since Illya still seems to have that inside her. She's more capable than Illya in many things...but also very logical in everything (that gets in her way early on, with the whole flying thing).

So my guess is she is an alternate universe version of Illya. Probably not the Fate/Prototype version (since the manga writer wouldn't have know about that), but some other variant of Illya that Illya and Kero are not.

One thing that is odd is that the story seems to take place about when Fate/Stay Night's story would have happened, but Illya seems too young in this manga based on how she looks in Fate/Zero. Yet everyone else seems the correct age. She has the age that she appears, rather than her actual age. At least from my understanding of things.