Alfred and Amelia orphans and siblings momence, it's fun thinking about them as twins or as Alfred as an older sibling. It gives them both different vibes fr. If they're twins, I can see them being attached at the hip, like, the world may not want us but we have each other. Amelia gets on him for being impulsive but she's literally just as impulsive, and also ride or die for him. But that's fine because he's ride or die for her. Loyalty is coming up as a theme a lot I've noticed and I think in this au Alfred is loyal to Amelia first and everyone else second. Even when he kind of gets adopted into and folded into the FACE fam his heart is still with his sister, for better and for worse.
In scenarios where she's younger than him I can only see this being doubly so. His hero complex coming from him always wanting to protect his sister perhaps? I can see it happening both if they're twins and if Amelia is a bit younger than him but I feel like the feeling of "I need to protect this person" would be enhanced from years of experience of him kind of taking on a guardianship role around her, even if he was like. Six. When it started.
How long were they in the foster care system? Perhaps they were very young when they entered the system but still just barely old enough to remember having some kind of parent before all of that happened. Maybe Alfred's memories of that parent bleeds into his flashback memories of Oliver when that starts getting relevant. I can see them ping ponging around homes for a bit in the beginning and then stabilizing when they're teenagers, but by that point the two only see each other as family, even if they trust the person taking care of them. In my mind the npification happens when Alfred is nineteen but the alternative of it happening when he's sixteen is also super compelling because this poor boy JUST got something semi-permanent in his life and now this shit happens oops. Anyway regardless of how you spin it I think they both work at Taco Bell and cargate happens when Alfred is driving them both home