I agree–[name_m]Knox[/name_m] and [name_u]Lennox[/name_u] are way too close for me. Sorry! What about something like [name_f]Eleni[/name_f] or [name_f]Lena[/name_f]? Has the [name_m]Len[/name_m]- from [name_u]Lennox[/name_u], but not the repeat of the -nox ending? I think [name_f]Eleni[/name_f] [name_f]Jane[/name_f], [name_f]Elena[/name_f] [name_f]Jane[/name_f], or [name_f]Lena[/name_f] [name_f]Jane[/name_f] would all be gorgeous!
In my opinion, [name_m]Knox[/name_m] and [name_u]Lennox[/name_u] definitely sound way too close to be sibling names. You didn’t ask for recommendations but I couldn’t resist; sorry if this isn’t what you wanted!
If you like surname-type names for girls, some of my suggestions would be:
[name_u]Hollis[/name_u]
[name_u]Sutton[/name_u]
[name_u]Blair[/name_u]
[name_u]Adair[/name_u]
[name_u]Bellamy[/name_u]
[name_u]Emery[/name_u]
[name_u]Everly[/name_u]
[name_f]Harlow[/name_f]
[name_u]Marlowe[/name_u]
[name_u]Sawyer[/name_u]
[name_u]Harper[/name_u]
[name_u]Piper[/name_u]
[name_u]Raleigh[/name_u]/[name_u]Riley[/name_u]
[name_u]Kennedy[/name_u]
[name_f]Paige[/name_f]
[name_u]Hadley[/name_u]
[name_u]Sloane[/name_u]
If the L-syllable combined with the contemporary streamlined sound was what drew you to [name_u]Lennox[/name_u], here are some others:
[name_f]Laurel[/name_f]
[name_u]Laine[/name_u]/[name_u]Lane[/name_u] ([name_f]EDIT[/name_f]: oops! if middle name is [name_f]Jane[/name_f], this would be too rhyme-y)
[name_f]Linnet[/name_f]
[name_f]Lena[/name_f]
[name_u]Lyric[/name_u]
[name_u]Linden[/name_u]
Lakelyn
Lumi
[name_f]Lola[/name_f]
[name_f]Lula[/name_f]
[name_f]Liv[/name_f]/[name_f]Livia[/name_f]
My top recommendations with [name_m]Knox[/name_m] and [name_m]Fitz[/name_m] though, would be [name_f]Lux[/name_f] or [name_f]Liv[/name_f]. Both would keep that one-syllable, modern-sounding pattern with your other kids’ names. [name_f]Lux[/name_f] still sounds similar to [name_m]Knox[/name_m], but isn’t way too close to be unusable. [name_f]Liv[/name_f] sounds distinct, and also has a unique letter in the name (v) to match [name_m]Knox[/name_m] (x) and [name_m]Fitz[/name_m] (z).
Sorry, I have to agree that [name_m]Knox[/name_m] and [name_u]Lennox[/name_u] are way too close. Seems like you really like that “ox” element though, so how about [name_f]Roxanne[/name_f]? You could call her [name_f]Roxy[/name_f] for short. It would be similar to [name_m]Knox[/name_m]'s name but not actually contain his name.
I think it would get very confusing to have [name_m]Knox[/name_m] and [name_u]Lennox[/name_u]. People might think [name_m]Knox[/name_m] was a nn for [name_u]Lennox[/name_u] as they are so similar.