I would probably go with
Abraham, but
Archibald is a better fit with
Theodore,
Ignatius,
Daphne, and
Winifred. I'm not too keen on the -bald part of
Archibald. I used to not care for
Archie, but it has grown on me some to the point that I find it somewhat endearing. If you'd like a less popular nickname, how about
Archibald "Archer"?
As for
Abraham, I agree that it sounds very grown up. I don't really care for the nickname
Abe, but I love
Bram as either a full name or nickname.
Abram is also a good nickname option, but you may consider using
Abram as a full name. It would work with the possible sibling names.