[name]Welcome[/name]!
Because the new baby’s siblings names are on the more modern side, I think [name]Waverly[/name], [name]Cambrie[/name], and [name]Hadley[/name] would all work great! Old-fashioned [name]Lorelai[/name] and [name]Felicity[/name] seem a little out of place in comparison, but I think [name]Felicity[/name] could still work well (I also just [name]LOVE[/name] [name]Felicity[/name], haha). I think [name]Synclair[/name] sounds way too masculine (even with the Y, which looks odd to me). Though pretty, [name]Arden[/name] and Darienne sound too much like [name]Donovan[/name] to my ear, while Aubryn and [name]Shealyn[/name] seem a little matchy. I really think [name]Waverly[/name] is a FANTASTIC option–crisp, but feminine and soft like [name]Shea[/name]; unusual, but with a spelling most people will get right. I think it will age well and has some cute nicknames ([name]Ava[/name], [name]Vera[/name], Wave, [name]Lee[/name]). I would probably opt for [name]Cambria[/name] and use [name]Cambrie[/name] as a nn, but it’s sweet, too. And [name]Hadley[/name] would work great with her siblings, but won’t be as unusual. Where [name]Waverly[/name] hasn’t registered in the Top 1000 yet (according the Social Security Admin), [name]Hadley[/name] is ranked at 216 or so. I don’t think any girl will begrudge being called any other these three options though!
With [name]Shea/name and (Dono)[name]Van[/name], I think [name]Blaise[/name], [name]Weston[/name], and [name]Cannon[/name] work best. [name]Vaughan[/name] is way too similar to [name]Van[/name] sound-wise. [name]Kasen[/name] looks made-up to me, like a trendy mash-up of [name]Kasey[/name] and [name]Jason[/name]. Sorry! I also confess that Treyson looks off to me, too. I like [name]Trey[/name] or [name]Trace[/name] all right. I would recommend [name]Greyson[/name] instead, but [name]Shea[/name] and [name]Grey[/name] rhyme (as would [name]Trey[/name]). That would bother me (maybe you don’t mind, though). [name]Ryland[/name] looks like a great option, but something keeps tripping me up. Maybe it sounds or is structured too much like [name]Shealyn[/name]? Similarly, [name]Lincoln[/name] is one of my favorite names, but it sounds a lot like [name]Shealyn[/name] to my ears. I love that [name]Blaise[/name] has a different sound & ending from [name]Shealyn[/name] and [name]Donovan[/name] to give your bunch more variety. My only concerns are that it could be a tad feminine. I think [name]Weston[/name] shares the solid, slightly rugged feeling of [name]Donovan[/name]; [name]West[/name], [name]Shea[/name], and [name]Van[/name] sound great together. This is my favorite of your choices. [name]Cannon[/name] and [name]Donovan[/name] are a tad similar, but I think it manages to work. They sound like brothers to me. 
All three are also unusual, but simple to spell.
I think you’ve got terrific options for girls; I might offer [name]Kinsey[/name] and [name]Ainsley[/name] up as suggestions similar to [name]Waverly[/name]. For boys, I would suggest looking at a few more names that don’t end in -n (not that you have to avoid -n names, of course, but I think a different ending could mix things up nicely). [name]Shea[/name] and [name]Donovan[/name] are both Irish, so you might check out some more Irish kinds of names, too. What about [name]Brady[/name] or [name]Brody[/name], [name]Garrett[/name], [name]Rory[/name], [name]Roarke[/name], [name]Rafferty[/name], [name]Beckett[/name], or [name]Finley[/name]? Perhaps [name]Blake[/name] would be an interesting alternative to [name]Blaise[/name] (less fem. and it doesn’t mean “to lisp, stutter”). And I think [name]Sinclair[/name] would be great on a little boy!
Congrats, and good luck!