I don't mind
August for a girl. I think it has a masculine enough sound that it isn't going to "switch" any time soon and it's the name of a month as well, which definitely makes it more unisex. It has a similar feel to
River.
I too would have preferred
Augusta but I think the flow of
August Zuckerberg is better than
Augusta Zuckerberg. I hope they call her
Gussie or Augie as a nickname.
Max and
Gussie sound very cute as sisters.