Love Augustine. I recently met a little
Augustine, about three, who had a brother named
Hamilton. I think such it's formal while still having an air of youth around it. It still needs a less cooped-up middle though, to balance it out. From yours, I think these are great:
Augustine AdeleAugustine BeatrixAugustine BlytheAugustine GeorgiaAugustine HazelAugustine PearlAugustine Sylvie
Beatrice and
Marguerite are a little too similar for the same name (over three syllables, I-consonant-E endings) but they would make a fantastic sibset.
Beatrice,
Augustine, and
Marguerite. Classics with a French nod.
I also think
Augustine Ruby would be lovely, or depending on how you feel on two middles:
Augustine Ruby Celeste,
Augustine Ruby Marguerite, or
Augustine Alice Pearl.