I think Juliette goes the best with your other two daughters followed by Olivia and then Corinne. The other two are a different style the the first 2 in my opinion. All beautiful names though. 

Juliette seems the best fit to me, and if it would honor your mil, all the better.

Olivia would be my second choice. It is quite popular. Only you can decide if it's too much so!

Corinne seems not quite the same style, though I think it could work, and it's lovely and uncommon.

London doesn't appeal to me and seems like a style mismatch.

Ivy is a darling name but doesn't fit naturally with the others for me.

I think Marie works fine with each of them.

I agree with the others.  While all of your choices are lovely names, I think Juliette sounds best with Elliana & Scarlett. 

I love Juliette, but I think the ending is too close to Scarlett to make it a good fit. Corinne is my favorite in this case—both its beginning and ending are distinct from the other two names, and it has a sleek and sophisticated feel to it that pulls together the more frilly style of Elliana and the more edgy style of Scarlett. Olivia is nice, but I do think that it's a bit overplayed these days. London is not normally a style I care for; I think it matches Scarlett better than Elliana. Ivy is another lovely name that I don't think fits so well, particularly because Scarlett and Ivy together sound a bit too descriptive. 

I like Ivy, I just don't think it's a great fit with Elliana & Scarlett.

I don't particularly care for London, and I think it is a dramatic depature style-wise.

Juliette, I really like and it is my favorite of your 3 top choices.  However, I agree that I'm not crazy about the 2 "ett" endings. Julia could be a good option, if you like it.

Olivia I like, but it is very common.  I think it is a very good match for Elliana but not so much for Scarlett.

Corinne is my favorite from your list.  It has the length of both Elliana & Scarlett, matches them both for femininity, all while having a sound that is distinct from the sister names.  I think it is the best for something to bridge the styles of Elliana & Scarlett.

I know you didn't ask for more suggestions, but I did think of Jocelyn.  Same ending sound as Corinne and keeps the J first initial to match your MIL's.

Ivy--love the name, but I don't think it fits well with your girls' names.

London--can't even say I like it, but again, it doesn't really "go" with her sisters.

Juliette--Love it stylistically with her sisters, but hesitant about the two "ett(e)" sounds

Olivia--Pretty, fits stylistically, might be too common. If you loved it, I would say go for it, but as it's not high on the list, maybe not.

Corinne--This fits nicely, has its own sounds, and I love that they would all have a different double consonant in their names. I also like Corinna (if you're going for the -een sound of this name) to bridge the 4 syllable and 2 syllable names. :-)

I'll disagree with some of the previous posters and say that I think Ivy works well with the other 2 names. I think it is probably one of the best matches.

I love Juliette and that would be my top pick, except 2 'et' endings leaves one girl out. However, with my accent I pronounce Juliette like JOOL-ee-ET and Scarlett like SCAR-lit so they don't actually match. The 'et' is definitely more pronounced with Juliette than Scarlett. If the similar/matching endings don't bother you then I'd go with Juliette. I think it's fabulous and a nice tie in if it gives the bub the same initials as your MIL.

Corinne would be my third pick. I like it, but just prefer the other two more.

London I don't mind but it's very different stylistically from your other daughters and other top choices. I wouldn't pick it because of that.

Olivia actually fits in quite well but is very popular. Ell/Elly names and Scarlett are relatively popular in my area so a sister named Olivia wouldn't be that unusual but it depends on whether it bothers you that it would be more popular than the other names.  Have you considered Olive? I think that would fit well, is less popular and Olive Marie is lovely.

I think they all work well with the middle name Marie, so no dramas there!

Wow, you have an incredibly similar naming style to my sister's.  She has an Elliana and her final daughter's middle name is Scarlett. I think she would have led with Scarlett, but by then she had a theme going and didn't want to break it. Her other names are Cecilia, Alma and Katarina, so perhaps those may interest you?  I think Ivy, Olivia and Corinne all go beautifully with your other girls names. As the mother of an Ivy, I can endorse the name completely.  People love it, and it's a name that is at home anywhere. Olivia is a bit common, but I don't really think that should be a dealbreaker anymore. Common names just aren't THAT common anymore (unless it is in your area. . .only you can answer that though).  And Corinne is lovely. It's unique and unexpected and a perfect match to the other two.  London feels like an outlier to me, and Juliette rhymes with Scarlett, which may bother Elliana at some point.  

Hmm.  As another mother of an Ivy, I'd say I think it goes well enough with Scarlett, but is very different from Elliana.  Elliana is very frilly and has that romance language flair.  Ivy is more Anglo and serious and/or spunky.  Scarlett is midway between the two, for me, having a romantic and feminine feel due to Scarlett O'Hara and the like, but has the crisp/non-frilly -t ending.  I would pick Juliette, Corinne, and Olivia as the best match for your first two.  I agree with the others that London is an outlier and does not really match the other two welll at all. 

Corinne would be my first pick from your list, because I think it is pretty and underused, and is a nice style bridge between your other two (softer and more continental than Scarlett, but less frilly than Elliana).  Juliette fits this bill too, but having two girls with -t endings might bother you.  It seems ok to me, since I say SCAR-let and ju-lee-ETTE, and the difference in emphasis makes the names not sound too similar, but it sounds like that may vary from person to person.  Olivia works fine as well, but is too common for my liking these days.  A few other suggestions that came to mind, in case you are still looking for options: Jocelyn, Camille/Camilla, Cecelia/Cecile/Cecily, Vivian, Lillian, Adele/Adela. 

I'm going to disagree with most and say that London is a good choice!  It's not really my style, and I would agree it's stylistically different than Elianna and Scarlett, but I kind of like that in this case.  I think a little variability in style can be lovely.  Same goes for Ivy, so I would say they are my two favorites.  They give the sibset as a whole a little more edge.

Juliette and Olivia are also both nice but to me push the sibset into the frilly realm more than I like.  I'd probably prefer Olivia over Juliette cause of the repeating endings with Scarlett and Juliette (although that's not too big of a deal to me for the reasons others stated.)

This might be picky, but I don't really like Corinne because of how Corinne and Marie both have strong accents on the second syllable... it' too sing-songy, or something.  Can't quite put my finger on it.  Although I guess the French do it all the time. 

I like Corinne Marie best (I know someone who has a little Corinne Marie).

Of your bolded choices I like Juliette Marie, then Ivy Marie.

I personally don't really like names of places used as baby names, unless there is a specific connection to the place.