One example of
Austin as another name for
Augustine is in
Geoffrey Chaucer's
The Canterbury Tales, when the narrator references
Austyn, meaning St.
Augustine.
Honestly, it's not my favorite. I feel like it's a little past it's prime right now, having peaked in the early 90s. I don't think the sound is especially pleasing, and I'd think of the city in Texas before
Augustine.
Also, I'm not a fan at all of themed initial sets for siblings. Anrew and
Austin would be entirely too close for brothers. I would probably try to avoid anything that starts with A since your son is
Andrew, to be honest.
Augustine itself isn't so bad (if not for pairing it with
Andrew), but I actually really like
Augustus.
Wit has truth in it; wisecracking is simply calisthenics with words.