We are so indecisive and looking for a name for little brother for older sisters [name_f]Sydney[/name_f] and [name_f]Margot[/name_f]. We would sincerely appreciate your insight and expertise!
Names we currently like (but aren’t sold on) are:
[name_m]Felix[/name_m]
[name_m]Julian[/name_m]
[name_m]Simon[/name_m]
[name_m]Hugh[/name_m]
[name_m]Conrad[/name_m]
[name_m]Lewis[/name_m]
[name_m]Clark[/name_m]
[name_m]Archie[/name_m]
[name_m]Rowan[/name_m]
I like [name_m]Archie[/name_m] and [name_m]Lewis[/name_m] from your list! with [name_m]Lewis[/name_m], each sibling would have their own ending though.
From this list I would definitely go with [name_m]Julian[/name_m], [name_m]Felix[/name_m] or [name_m]Lewis[/name_m].
I love [name_m]Julian[/name_m], [name_m]Archie[/name_m], & [name_m]Clark[/name_m] with [name_f]Sydney[/name_f] & [name_f]Margot[/name_f]!
Other ideas:
[name_m]Alexander[/name_m]
[name_m]Auggie[/name_m]
[name_m]Blaine[/name_m]
[name_m]Bruce[/name_m]
[name_m]Charles[/name_m]
[name_m]Charlie[/name_m]
[name_m]Christian[/name_m]
Clarkson
[name_m]Connor[/name_m]
[name_m]David[/name_m]
[name_m]Glen[/name_m]
[name_m]Grayson[/name_m]
[name_m]Harrison[/name_m]
[name_m]Irvin[/name_m]
[name_m]Jack[/name_m]
[name_m]James[/name_m]
[name_m]John[/name_m]
[name_m]Lawson[/name_m]
[name_m]Nelson[/name_m]
[name_m]Paul[/name_m]
[name_m]Theodore[/name_m]
[name_m]Tom[/name_m]
[name_m]Xavier[/name_m]
[name_m]Walter[/name_m]
[name_m]William[/name_m]
welcome to nameberry!
I think my favorite from the names you listed is [name_f]Sydney[/name_f], [name_f]Margot[/name_f] & [name_m]Lewis[/name_m] 
[name_m]Simon[/name_m] & [name_m]Rowan[/name_m] work well too!!
I would recommend a two syllable name with emphasis on the first syllable and/or a name that ends in a vowel sound. However, all the names you chose have a lot of charm and potential.
Here are some extras that come to mind:
[name_f]Sydney[/name_f], [name_f]Margot[/name_f] & [name_m]Lucas[/name_m] or [name_f]Sydney[/name_f], [name_f]Margot[/name_f] & [name_m]Luca[/name_m]
[name_f]Sydney[/name_f], [name_f]Margo[/name_f] & [name_m]Levi[/name_m]
[name_f]Sydney[/name_f], [name_f]Margot[/name_f] & [name_m]Jonah[/name_m]
[name_f]Sydney[/name_f], [name_f]Margot[/name_f] & [name_m]Leroy[/name_m]
[name_f]Sydney[/name_f], [name_f]Margot[/name_f] & [name_m]Henry[/name_m]
[name_f]Sydney[/name_f], [name_f]Margot[/name_f] & [name_m]Benji[/name_m]
[name_f]Sydney[/name_f], [name_f]Margo[/name_f] & [name_m]Rory[/name_m]
[name_f]Sydney[/name_f], [name_f]Margot[/name_f] & [name_m]Corey[/name_m]
others:
[name_m]August[/name_m]
[name_m]Silas[/name_m]
[name_m]Judah[/name_m]
[name_m]Calvin[/name_m]
[name_m]Colby[/name_m]
[name_m]Ellis[/name_m]
[name_m]Harris[/name_m]
[name_m]Malcolm[/name_m]
[name_m]Kenneth[/name_m]
[name_m]Jaden[/name_m]
[name_m]Cyrus[/name_m]
[name_m]Beauden[/name_m]
[name_m]Owen[/name_m]
[name_m]Collin[/name_m]
[name_m]Sonny[/name_m]
[name_m]Rigby[/name_m]
[name_m]Ozzy[/name_m] or [name_f]Ollie[/name_f]
[name_m]Finley[/name_m]
[name_m]Lenny[/name_m]
[name_m]Bowie[/name_m]
[name_m]Toby[/name_m]
[name_m]Koda[/name_m] or [name_m]Koa[/name_m]
[name_m]Noah[/name_m]
[name_m]Lawson[/name_m]
[name_m]Bodie[/name_m]
[name_m]Amos[/name_m]
From your list I think [name_m]Clark[/name_m] or [name_m]Conrad[/name_m] would work really well 
Some suggestions, sorry for any repeats…
[name_m]Bryce[/name_m]
[name_m]Scott[/name_m]
[name_m]Calvin[/name_m]
[name_m]Daley[/name_m]
[name_m]Christopher[/name_m]
[name_m]Callum[/name_m]
[name_m]Davis[/name_m]
[name_m]Seth[/name_m]
[name_m]Weston[/name_m]
[name_m]Bradley[/name_m]
[name_m]Rhys[/name_m]
[name_m]Eric[/name_m]
[name_m]Stefan[/name_m]
[name_m]Zachary[/name_m]
[name_m]Flynn[/name_m]
[name_m]Aaron[/name_m]
[name_m]Max[/name_m]
[name_m]Greyson[/name_m]
[name_m]Dwayne[/name_m]
[name_m]Ryan[/name_m]
[name_m]Caleb[/name_m]
[name_m]Leon[/name_m]
[name_m]Benjamin[/name_m]
[name_m]Gabriel[/name_m]
[name_m]Kobe[/name_m]
[name_m]Wyatt[/name_m]
[name_m]Lawrence[/name_m]
[name_m]Travis[/name_m]
[name_m]Samuel[/name_m]
[name_m]Harvey[/name_m]
[name_m]Nathan[/name_m]
[name_f]Sydney[/name_f] and [name_f]Margot[/name_f]
I do like:
[name_m]Felix[/name_m] - light, bright and cheerful!
[name_m]Conrad[/name_m] - unexpected, cool and charming
[name_m]Julian[/name_m] - international with flair and solidness too
[name_m]Rowan[/name_m] - warm, lovely and gentle but outdoorsy too
I’ll suggest:
[name_f]Sydney[/name_f], [name_f]Margot[/name_f] and…
[name_m]Beckett[/name_m]
[name_m]Rory[/name_m]
[name_m]Linus[/name_m]
[name_m]Jasper[/name_m]
[name_m]Tate[/name_m]
[name_m]Jude[/name_m]
[name_m]Adrian[/name_m]
[name_m]Alec[/name_m]
[name_m]Ralph[/name_m]
[name_m]Otto[/name_m]
[name_m]Rhys[/name_m]
[name_m]Jonah[/name_m]
[name_m]Robin[/name_m]
[name_m]Ellis[/name_m]
[name_m]Flynn[/name_m]
[name_m]Franklin[/name_m]
[name_m]Ralph[/name_m]
[name_m]Callum[/name_m]
[name_m]Paxton[/name_m]
[name_m]Hugh[/name_m] is perfect! I also like [name_m]Julian[/name_m], [name_m]Clark[/name_m], and [name_m]Conrad[/name_m] for your style. I’d nix [name_m]Simon[/name_m] not to have two sibs w same letter and one without. [name_m]Feliz[/name_m] doesn’t feel quite right.
I’d add [name_m]Preston[/name_m], [name_m]Lawrence[/name_m], [name_m]Calvin[/name_m], [name_m]Nelson[/name_m] or [name_m]Nolan[/name_m], [name_m]Bryant[/name_m], [name_m]Warren[/name_m]
From your list, I think [name_m]Simon[/name_m], [name_m]Rowan[/name_m], [name_m]Lewis[/name_m], [name_m]Julian[/name_m], [name_m]Archie[/name_m], and [name_m]Conrad[/name_m] would all work well.
Some other ideas:
[name_m]Jasper[/name_m]
[name_m]August[/name_m]
[name_m]Miles[/name_m]
[name_m]Logan[/name_m]
[name_m]Xavier[/name_m]
[name_m]Victor[/name_m]
[name_m]Bennett[/name_m]
[name_m]Beckett[/name_m]
I’ve always loved [name_m]Julian[/name_m] with the nn [name_m]Jules[/name_m]. [name_m]Hugh[/name_m] is sweet but I prefer [name_m]Hugo[/name_m].
Great choices. I think [name_m]Felix[/name_m], [name_m]Julian[/name_m] and [name_m]Hugh[/name_m] go best.