Charles, hands down.
Charlie is a nickname, and has no formal name quality to it in the least. It's immature sounding and wouldn't age well. Plus, confusion could occur.
I don't care for the name
Max, again, because it's a nickname, but I don't care for any
Max- related names, either.
And, I think all names work in a sibling set, as long as they don't form a theme.
Flux-capacitor...fluxing... - "Back to the Future"