Here are some of the names we are considering. Any likes, dislikes (and why) anymore ideas along these lines.
[name]Elliot[/name]
[name]Campbell[/name]
[name]Harper[/name]
[name]Meriall[/name]
[name]Grey[/name]
[name]Layne[/name]
[name]Marin[/name]
[name]Quinn[/name]
[name]Sawyer[/name]
[name]Flynn[/name]
I really like [name]Marin[/name]!
I like [name]Harper[/name], but it is a very trendy name where I live.
I am not sure what to think of [name]Meriall[/name]…how is that pronounced?
Honestly, I can’t picture [name]Elliot[/name], [name]Campbell[/name], [name]Grey[/name], [name]Lane[/name]/[name]Layne[/name], [name]Quinn[/name], [name]Sawyer[/name], or [name]Flynn[/name] on a girl…they scream “boy” to me. Sorry! I guess I see [name]Logan[/name] as a “softer” boy name (I love the name, by the way!), and I think these names are great with [name]Logan[/name], but for [name]Logan[/name]'s brother 
I [name]LOVE[/name] the names [name]Elliot[/name] and [name]Quinn[/name], but to me they aren’t feminine/girly enough to use them on a girl. Of course this is just my opinion 
I have always liked [name]Laina[/name]/[name]Layna[/name] – nickname [name]Layne[/name]/[name]Laney[/name]; that would be an option if you want to make [name]Layne[/name] more feminine.
I’ll put them in order from favorite to least favorite.
[name]Marin[/name]: Sweet and soft, goes well with [name]Logan[/name].
[name]Harper[/name]: A little trendy but still really nice with [name]Logan[/name].
[name]Quinn[/name]: Very cute.
[name]Layne[/name]: Not a fan of y’s but [name]Layne[/name] is soft and sounds classy to me.
[name]Grey[/name]: I like it but feel like [name]Grey[/name] would be more fitting on a boy.
[name]Meriall[/name]: Idk how to pronounce this name.
[name]Sawyer[/name]: Another name I like on a boy.
[name]Flynn[/name]: I don’t like this for a boy or girl and it sounds too much like [name]Finn[/name] which is getting popular.
[name]Elliot[/name]: I love this name, on a boy.
[name]Campbell[/name]: I just don’t find this appealing.
[name]Hi[/name]!
[name]Elliot[/name] - I like this more for a boy. [name]Ella[/name], [name]Elise[/name], or [name]Ellis[/name] would be better for a girl!
[name]Campbell[/name] - Far too masculine for my taste, especially alongside [name]Logan[/name]. [name]Camille[/name], maybe? Or even [name]Cameron[/name]?
[name]Harper[/name] - This is ideal. My favorite so far off your list! Feminine but still vaguely unisex, perfect with [name]Logan[/name]…
[name]Meriall[/name] - I’ve never heard of this name. I think you’ll run into trouble with spelling and pronunciation, for sure.
[name]Grey[/name] - Eh, this is iffy for me. But, alas, I feel it is more suited to a middle name. Reminds me of gray days!
[name]Layne[/name] - I’m not a fan of siblings sharing the same first initial, and this feels slightly dated to me. [name]Lacey[/name], if you must?
[name]Marin[/name] - [name]Logan[/name] and [name]Marin[/name] both end in the same sound and they feel different, style-wise. Like, but don’t love.
[name]Quinn[/name] - [name]Adore[/name] [name]Quinn[/name] for a girl, but it sounds similar to [name]Logan[/name], so I wouldn’t use it…
[name]Sawyer[/name] - This is quirky and cool, but it is one that goes either way in terms of girl or boy. I’m feeling boy with [name]Logan[/name].
[name]Flynn[/name] - [name]Way[/name], way, way too masculine! With [name]Campbell[/name], this would make an awesome set of brothers.
So, hands-down, I say you should use [name]Harper[/name] with [name]Logan[/name] - it’s perfect!
Others you might like are [name]Afton[/name], [name]Alcott[/name] “[name]Allie[/name],” [name]Blair[/name], [name]Delaney[/name] “[name]Laney[/name],” [name]Ellis[/name] “[name]Elle[/name],” [name]Hollis[/name] “[name]Holly[/name],” [name]Jessa[/name], [name]Laurel[/name], [name]Macy[/name], [name]Marley[/name], [name]Payton[/name], [name]Paige[/name], [name]Piper[/name], [name]Reese[/name], [name]Tess[/name], and [name]Tierney[/name].
[name]Lemon[/name] 
I agree with [name]Lemon[/name] that [name]Harper[/name] is the best with [name]Logan[/name]. I think the others are too unisex to work as a sister to [name]Logan[/name], and [name]Maren[/name] is to rhymey. Other surname-types that are more clearly feminine…
[name]Avery[/name]
[name]Piper[/name]
[name]Delaney[/name]
[name]Tierney[/name]
[name]Ellery[/name]
[name]Tatum[/name]
[name]Greer[/name]
[name]Cassidy[/name]
Or what about changing Merriall to [name]Merrill[/name]? Or [name]Maris[/name]?
[name]Harper[/name] would be the best choice to pair with [name]Logan[/name]. I like [name]Marin[/name] and [name]Quinn[/name] but they have similar sounds as your daugter’s name so I wouldn’t choose them. Here are some other suggestions.
[name]Logan[/name] and sister…
[name]Aubrey[/name]
[name]Carey[/name]
[name]Chauncey[/name]
[name]Darcy[/name]
[name]Delaney[/name]
[name]Finley[/name]
[name]Grier[/name]
[name]Hadley[/name]
[name]Hailey[/name]
[name]Hollis[/name]
[name]Kendall[/name]
[name]Kinsey[/name]
[name]Macy[/name]
[name]Shea[/name]
[name]Schuyler[/name]
[name]Shelby[/name]
[name]Tierney[/name]
[name]Vaughan[/name]
You definitely like the gender-neutral names, which I tend to like a lot too (my own list doesn’t reflect that, but I still love gender-neutral names in general).
[name]Elliot[/name] - I like it a lot on a girl, even more than a boy. There are a host of spelling choices, [name]Elliott[/name] being my personal fave, but [name]Elliot[/name] and [name]Eliot[/name] are nice as well. [name]Eliot[/name] has a bit more masculine feel for me, for whatever reason.
[name]Campbell[/name] - I know a lot of people with this last name, so it’s hard to picture it as a first. I can see the appeal, though, especially if you’re looking for an unconventional route to [name]Bell[/name] or [name]Bella[/name].
[name]Harper[/name] - Nice, though trendy (doesn’t have to be a bad thing). Nickname potential is limited to “Harpy,” which is perhaps not the nicest nickname.
[name]Meriall[/name] - I would pronounce this [name]MARE[/name]-ee-all, is that what you’re going for? It reminds me of [name]Muriel[/name].
[name]Grey[/name] - Feels upper-crust, but grey by nature insinuates “drab.”
[name]Layne[/name] - I know too many guys named [name]Lane[/name]!! [name]Layne[/name] on a girl I think might cause confusion with [name]Elaine[/name], or “what’s it short for?” syndrome ([name]Alayne[/name], Jolayne, [name]Charlane[/name], [name]Marlena[/name]).
[name]Marin[/name] - [name]MARE[/name]-inn or muh-RINN? I associate it with [name]Marin[/name] County (suburb of San [name]Francisco[/name]), which is pronounced muh-RINN. If you’re looking for the [name]MARE[/name]-inn pronunciation, I think [name]Maren[/name] is the way to go.
[name]Quinn[/name] - [name]Love[/name] it. [name]May[/name], again, fall victim to “what’s it short for?” syndrome.
[name]Sawyer[/name] - I love this one for a girl. Some may argue it’s a masculine name, but for me, it’s either a last name or a feminine first name… it doesn’t work as a masculine fn for me!!
[name]Flynn[/name] - This is ALL surname to me!
Nearly all of the names on your list could be misconstrued as male names, which is my only concern because [name]Logan[/name] is also such a unisex name. It would be very hard to tell the gender of both children on paper. [name]Marin[/name] and and [name]Harper[/name] are the only names feminine enough to stand next to a male [name]Logan[/name] in my opinion. I prefer [name]Harper[/name] with [name]Logan[/name] because as others have mentioned [name]Marin[/name] has a slightly similar sound to [name]Logan[/name].