nymbler.com is a good place to start when trying to find sibling names!
Because the daughter's name is Emerson, I'd recommend sticking to more unisex names for any future daughters (or at least not naming the next one something overly feminine and frilly).
I like Brynn, Eden, Tamsin, and Ellery. Basically, I love all of your girl name options. I totally agree with the unisex, not frilly thing. If you name one girl a unisex name, you aren't likely to name the next one Talullah or anything like that.
Aubrey-strikes me as a tomboy, not too girly name, but that's a matter of opinion
Charlie-unisex and yet, when applied to a girl, an edge of feminine with a tomboy personality
Magnolia-unusual, but awesome, because it leaves the nickname door wide open. Maggies? Nola? Nolia?
November/September/Tuesday-okay, I'll admit it, I love date names.
Wyatt-I dunno. I've suggested this for just about every thread requiring a guy name. Apparently Wyatt is my name of the day.
Calvin-random name, came to mind, wrote it down
August- see above, about date names
I absolutely love the name Rae for a girl. That's so awesome.