I love [name_f]Cecilia[/name_f]!
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It’s one of my favourites. The repeated ‘s’ and long ‘e’ sounds in it create such a musical effect I haven’t found in many other names. Interestingly, the exact same sounds which make [name_f]Cecilia[/name_f] feel so delicate and pretty also make her feel fierce and bold [name_f][/name_f]- there’s just something about it. That makes it a perfect name [name_f][/name_f]- one with flair, history, strength and feminity.
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It’s also one of those names everyone has heard, but I’ve only really seen it used in books [name_f][/name_f]- I would probably struggle to name any real people I know of, famous or otherwise, who are called [name_f]Cecilia[/name_f] 
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[name_f]Cecilia[/name_f] and [name_f]Isla[/name_f] are nice names for sisters! Obviously they are different styles, but they go together, I can totally see the appeal of both. They are sweet and lovely names; [name_f]Cecilia[/name_f] is classic and [name_f]Isla[/name_f] is more modern/on-trend, but [name_f]Isla[/name_f] is well-established enough (and the whole Isla/island thing makes it different from a lot of other popular names, imo) and [name_f]Cecilia[/name_f] fits in with the top names of today, so it doesn’t feel strange at all. I kind of like that both names end in ‘a’ [name_f][/name_f]- it sounds nice when you say it aloud [name_f][/name_f]- and I’m loving the contrast of [name_f]Cecilia[/name_f] being longer and [name_f]Isla[/name_f] being shorter. When you put these two names together, the set has a real sunny vibe. I think of beaches and the ocean.
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[name_f]Cecilia[/name_f] [name_m]Christiansen[/name_m] (or similar) would be a genuinely cool name to have. I like that it’s technically alliteration, but in fact the ‘c’ makes a different sound in the first name and surname. It reminds of [name_f]Caroline[/name_f] [name_f]Cecilia[/name_f] [name_f]Charlotte[/name_f], a combo which I’ve seen being thrown around on Nameberry
[name_f][/name_f] [name_f]Cecilia[/name_f] [name_m]Christiansen[/name_m] also just looks nice and kind of balanced (soft name vs sharp surname) on paper.
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[name_f]Lena[/name_f] strikes me as a bit incomplete, and a name like [name_f]Lena[/name_f] [name_m]Christiansen[/name_m] only heightens the difference [name_f][/name_f]- you’ve got a long and formal-ish surname and a first name that pales by comparison. [name_f]Emilia[/name_f] is fine, elegant and pretty in theory, but I’ve never actually liked the sound of it, or of [name_f]Amelia[/name_f]. I’ve seen [name_f]Amelia[/name_f] being described as ‘sticky’, and that about sums it up [name_f][/name_f]- there aren’t enough strong sounds to balance out the softness, so it just doesn’t sound memorable to me.
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This post got away from me a bit, but, yes [name_f][/name_f]- use [name_f]Cecilia[/name_f]
[name_f][/name_f].