These are great names! I find it hard to rate names individually, so all of my opinions relate to how I would view these names within the sibset. Here are my thoughts …
Cecilia … I like that this is a four syllable name like your other girls’ names. It feels at home in your sibset, though it sounds softer and has less pop than the other sibset names. Cecilia could be a little lost in the mix.
Coraline … This is a strong name that is bold and stately yet fun and comfortable to wear. The name looks and sounds distinct while being similar enough to your other children’s names. A Coraline would do well in this sibset!
Georgia … Beautiful Georgia is a name that, like Coraline, feels comfortable with a bit of a fashionable flair. She’s classic and not an antique. Plus she’s a visually striking written name!
Maisie … This name feels like a nickname especially when put in your sibset.
Olive … Short and sweet Olive is such a darling, but I think she’s too similar to Milo and too tiny for your girl sibset.
Scarlett … This name is always such a pop in any sibset. I don’t think it balaces quite as nicely within your sibset though.
Violet … This name can always hold its own, even though it is shorter than many of your other female names in your top picks and your sibset. She’s appealing to say and looks striking when written. She’s a more popular choice, but that’s the price of a short dazzler. I think a little Violet would be a little joy for her siblings.
Vivian … This is such a pretty and soft name. However, it feels a bit underwhelming in your sibset. Visually, I like the spellings “Vivianne” and “Vivienne” within your sibset.
Overall, I would say my favorites (in order) are Violet, Coraline, and Georgia. All of these names pair well with your sibset while not being too similar to your children’s names. I think Violet would be the most striking choice, Coraline would be the most quirky traditional pick that blends best with her sisters’ names, and Georgia would be the stellar wild card.