[name_m]Jack[/name_m] and [name_f]Charlotte[/name_f] would be a lovely sibling pair. I’ve never been an [name_f]Ella[/name_f] fan. It just lacks substance for me.
It sounds like [name_f]Ella[/name_f] is your first love, so I would have to go with [name_f]Ella[/name_f] [name_f]Grace[/name_f], although I do love [name_f]Charlotte[/name_f]!
Here is another thought: how about [name_f]Grace[/name_f] as a first name? [name_f]Grace[/name_f] [name_f]Charlotte[/name_f] is beautiful, and I love [name_f]Grace[/name_f] and [name_m]Jack[/name_m] together. 
If you’re concerned that [name_f]Ella[/name_f] is on the way out, why not name her [name_f]Elena[/name_f] and call her [name_f]Ella[/name_f]? [name_f]Elena[/name_f] is certainly not on its way out.
I prefer the name [name_f]Charlotte[/name_f] to [name_f]Ella[/name_f] however [name_f]Ella[/name_f] [name_f]Grace[/name_f] (with brother [name_m]Jack[/name_m]) is just gorgeous!
[name_m]Jack[/name_m] and [name_f]Charlotte[/name_f] are very similar… classic names currently experiencing a major surge in popularity. [name_f]Ella[/name_f] is experiencing the same surge, but wouldn’t be considered a classic and will sound much more dated in the future (as lovely as it is).
I went with [name_f]Charlotte[/name_f], I like them both but [name_f]Ella[/name_f] is a just little tired for me and I love the nn possibilities with [name_f]Charlotte[/name_f].
I must say I’ve never even noticed that [name_f]Charlotte[/name_f] has the word “harlot” in it, maybe because I don’t hear that word used very much where I’m from, I’ve known a few Charlottes and I don’t think any of them had to deal with the harlot connection.
Thanks for all the good suggestions and thoughts.