I'm assuming akiha was the person to first kill her brother. On a side note I still don't feel like I can fully trust Arcueid, however that might just be me being stubborn. I still want to know why shikki is so infatuated with her tho.
Decided to check out the manga adaptation because the remake of the original looks interesting. Never touched any of this or FGO, except for like a couple hours trying out melty blood.