I love your daughters' names!
Definitely
Caspian James for the boy.
Gabriel works but
Ashton is too generic, and
Alexander doesn't work when you already have
Alexandria.
For the girl, I'd have to cheat and make
Lydia Elizabeth (or maybe
Elizabeth Lydia).
Laura May is nice but it's on the cute side, and sounds too 'country' for the more elegant names of the sisters. Same with
Autumn.
Lydia Lucille is just too alliterative and matchy.