This is of course hypothetical but what names would you suggest as sisters to William and Henry? I am completely stuck when it comes to girls names, none seem to measure up to the timelessness of William and Henry 
EDIT: I can’t use the names Elizabeth or Katherine/Catherine because I have family members with those names, which makes me sad because I think those two names compliment William and Henry perfectly
[name_m]William[/name_m] and [name_m]Henry[/name_m] are classics. A few names that I think could work in a sibset are:
[name_f]Julia[/name_f]
[name_f]Adelaide[/name_f]
[name_f]Agnes[/name_f]
[name_f]Esther[/name_f]
[name_f]Caroline[/name_f]
[name_f]Diana[/name_f]
[name_f]Eliza[/name_f]
[name_f]Louisa[/name_f]
[name_f]Susanna[/name_f]
[name_f]Patience[/name_f]
[name_f]Lucy[/name_f]
[name_f]Lavinia[/name_f]
[name_f]Hester[/name_f]
[name_f]Georgiana[/name_f]
[name_f]Fanny[/name_f]
[name_f]Adelaide[/name_f]
[name_f]Charlotte[/name_f]
[name_f]Ingrid[/name_f]
[name_f]Margaret[/name_f]
[name_f]Louisa[/name_f]
[name_f]Juliet[/name_f]
[name_f]Eleanor[/name_f]
[name_f]Victoria[/name_f]
[name_f]Isabella[/name_f], [name_f]Isobel[/name_f]
[name_f]Rebecca[/name_f]
[name_f]Christabel[/name_f]
[name_f]Wilhelmina[/name_f] (on paper, it’s close to [name_m]William[/name_m], but it’s pronounced like villa-mena, so you should be OK)
[name_f]Evangeline[/name_f]
[name_f]Rosalind[/name_f]
[name_f]Miriam[/name_f]
[name_f]Celeste[/name_f]
[name_f]Elsa[/name_f]
[name_f]Violette[/name_f]
[name_f]Lydia[/name_f]
[name_u]Evelyn[/name_u]
[name_f]Bernadette[/name_f]
[name_f]Elspeth[/name_f]
[name_f]Virginia[/name_f]
[name_f]Josephine[/name_f]
I know siblings named [name_m]William[/name_m] and [name_u]Allison[/name_u]! [name_m]William[/name_m], [name_m]Henry[/name_m] and [name_u]Allison[/name_u] work!
Other ideas: [name_f]Lucy[/name_f], [name_f]Diane[/name_f], [name_f]Jane[/name_f], [name_f]Erin[/name_f], [name_f]Erica[/name_f], [name_u]Waverly[/name_u], [name_f]Kaitlyn[/name_f], [name_f]Sophie[/name_f] (not [name_f]Sophia[/name_f] because that feels more modern), [name_f]Jessica[/name_f], [name_f]Carolyn[/name_f].
[name_m]William[/name_m] and [name_u]Allison[/name_u] have both pretty much managed to stay near the top of the charts for the last 40 or so years (and idk about [name_m]Henry[/name_m]). Maybe check out names that are near the top (say in the 30s-40s?) and have been there for a long time.
I like the suggestions of [name_f]Caroline[/name_f], [name_f]Lucy[/name_f], [name_f]Charlotte[/name_f], and [name_f]Margaret[/name_f]. Here are some more girl names I think of as classic and timeless:
[name_f]Alice[/name_f]
[name_f]Mary[/name_f]
[name_f]Helen[/name_f]
[name_f]Ruth[/name_f] (my favorite classic girl name!)
[name_f]Anne[/name_f]
[name_f]Claire[/name_f]
[name_f]Emily[/name_f]
[name_f]Jane[/name_f]
[name_f]Laura[/name_f]
[name_f]Victoria[/name_f]
[name_m]William[/name_m], [name_m]Henry[/name_m], and…
[name_f]Victoria[/name_f]
[name_f]Caroline[/name_f]
[name_f]Eleanor[/name_f]
[name_f]Charlotte[/name_f]
[name_f]Margaret[/name_f]
[name_f]Mary[/name_f]
[name_f]Claire[/name_f]
[name_f]Alice[/name_f]
[name_f]Anne[/name_f] or [name_f]Anna[/name_f]
[name_f]Sarah[/name_f]
[name_f]Alexandra[/name_f]