My daughter [name]Felicity[/name] gets called [name]Lissie[/name] as a pet name but it’s just like calling a [name]Zachary[/name] “[name]Zacky[/name]” or something, it’s not what she would introduce herself as to anyone or anything (not that she can yet haha). [name]Eloise[/name] would go by [name]Elsie[/name] probably 80% of the time and it would be more official. She would get called [name]Lou[/name], [name]Ellie[/name], [name]Elle[/name], etc. as pet names (please no suggestions of just naming her [name]Elsie[/name] or of going by the full [name]Eloise[/name], this part is set in stone!) Would [name]Felicity[/name] and [name]Elsie[/name] be too close for sisters? If it makes a difference, siblings names are [name]Audrey[/name] [name]Cecilia[/name], [name]Jude[/name] [name]Alexander[/name], [name]Christian[/name] [name]Nathaniel[/name], [name]Violet[/name] [name]Annabelle[/name], and then [name]Felicity[/name] [name]Charlotte[/name] is the baby of the group. Thanks so much! 
Personally, I wouldn’t name her [name]Eloise[/name] if you’re so set on calling her [name]Elsie[/name] and her big sister’s name is [name]Felicity[/name]. [name]Felicity[/name] and [name]Eloise[/name] are great! [name]Felicity[/name] and [name]Elsie[/name] are way too similar. I just wouldn’t do it. All lovely names though!
As much as I like [name]Felicity[/name] and [name]Eloise[/name] together, I don’t think I would use [name]Eloise[/name] if you plan to use [name]Elsie[/name] as a nickname when you already have a [name]Lissie[/name].
It’s too ‘el’-y and ‘s’-y. They clash too much sorry.
But I must say [name]Audrey[/name] [name]Cecelia[/name] and [name]Jude[/name] [name]Alexander[/name]. Me oh my. 
First off, I love all your children’s name. Personally, I don’t think felicity and [name]Eloise[/name] are too close. Although I agree that the nicknames [name]Elsie[/name] and lissie are close, I don’t think that should matter because those are their nicknames and not their given names, so I wouldn’t worry too much about it.
I don’t think they’re too close at all. I don’t see a problem with [name]Felicity[/name] and [name]Eloise[/name], [name]Felicity[/name] and [name]Elsie[/name], or even [name]Lissie[/name] and [name]Elsie[/name] to be perfectly honest. [name]Eloise[/name] matches [name]Felicity[/name] and all your children’s wonderful names beautifully; the difference in length and style distinguish [name]Felicity[/name] and [name]Elsie[/name] from each other; and - maybe I’m just used to hearing lots of nicknames as they are so vastly popular here in the UK - [name]Lissie[/name] and [name]Elsie[/name] sound like two well-named, sweetly nicknamed sisters.
Good luck 
And I just have to say, your children’s names are stunning!