I absolutely love this name, and, although I won’t be having kids for at least 5 years (probably more like 10 years), I do love to plan out future sibling names. [name]Agatha[/name] is one that I’m really stuck on though. For a boy, I find that [name]Walter[/name], [name]Gabriel[/name], [name]Edward[/name] and [name]Henry[/name] match nicely, and [name]Ottilie[/name], [name]Beatrice[/name], [name]Florence[/name] and [name]Cecilia[/name] match well for girls, but I’d love to hear your ideas! 
[name]Beatrice[/name] and [name]Cecilia[/name] are lovely girl names. I am a HUGE [name]Henry[/name] fan, but [name]Walter[/name] is dashing as well.
Some other names I am kind of fond of are:
[name]Agatha[/name] & [name]Ingrid[/name]
[name]Agatha[/name] & [name]Matilda[/name]
[name]Agatha[/name] & [name]Evelyn[/name]
[name]Agatha[/name] & [name]Isis[/name]
[name]Agatha[/name] & [name]June[/name]
[name]Agatha[/name] & [name]Jasper[/name]
[name]Agatha[/name] & [name]Phineas[/name]
[name]Agatha[/name] & [name]Jude[/name]
[name]Agatha[/name] & [name]Lucas[/name]
[name]Agatha[/name] & [name]Tobias[/name]
[name]Agatha[/name] and
[name]Iris[/name]
[name]Pearl[/name]
[name]Alma[/name]
Coriander
I think all of your names are good siblings for [name]Agatha[/name]. Here are some other names that I think would be “natural” siblings.
[name]Agatha[/name] and sister…
[name]Edith[/name]
[name]Millicent[/name]
[name]Josephine[/name]
[name]Mabel[/name]
[name]Wilhelmina[/name]
[name]Susannah[/name]
[name]Eunice[/name]
[name]Miriam[/name]
[name]Winifred[/name]
[name]Henrietta[/name]
[name]Constance[/name]
[name]Eugenia[/name]
[name]Lucille[/name]
[name]Georgina[/name]
[name]Rosamund[/name]
[name]Cecily[/name]
[name]Philomena[/name]
[name]Lavinia[/name]
[name]Clementine[/name]
[name]Bernadette[/name]
[name]Estelle[/name]
[name]Claudine[/name]
[name]Sybil[/name]
[name]Philippa[/name]
[name]Thomasina[/name]
[name]Theodosia[/name]
[name]Harriet[/name]
[name]Eleanor[/name]
[name]Geraldine[/name]
[name]Agatha[/name] and brother…
[name]Theodore[/name]
[name]Laurence[/name]
[name]Edmund[/name]
[name]Clement[/name]
[name]Basil[/name]
[name]Constantine[/name]
[name]Horatio[/name]
[name]Dominic[/name]
[name]Ferdinand[/name]
[name]Benedict[/name]
[name]Oliver[/name]
[name]Sebastian[/name]
[name]Leopold[/name]
[name]Timothy[/name]
[name]Cornelius[/name]
[name]Humphrey[/name]
[name]Rufus[/name]
[name]Gregory[/name]
[name]Harvey[/name]
[name]Reuben[/name]
[name]Eugene[/name]
[name]Boniface[/name]
[name]Julius[/name]
[name]Rudolph[/name]
[name]Sylvester[/name]
[name]Hubert[/name]
[name]Cyril[/name]
[name]Gilbert[/name]
[name]Frederick[/name]
[name]Clarence[/name]
[name]Bernard[/name]
Let me just point out that I as well am not expecting but collecting names, and we share what seems like every single name! [name]Walter[/name], [name]Edward[/name], [name]Henry[/name], Beatric, [name]Cecilia[/name]. That being said, here are the one that go well I think
[name]Cordelia[/name]
[name]Violet[/name]
[name]Marion[/name]/[name]Marian[/name]
[name]Mathilde[/name]
[name]Greta[/name]
[name]Francis[/name]
[name]Victoria[/name]
[name]Sylvie[/name]
[name]Albert[/name]
[name]Hugh[/name]
[name]Willis[/name]
[name]Bertram[/name]
[name]Merritt[/name]
[name]Conrad[/name]