At the moment i have three combos that we both like:
[name_f]Nell[/name_f] [name_f]Josephine[/name_f] [name_u]Maria[/name_u]
[name_f]Juno[/name_f] [name_f]Elisabeth[/name_f] [name_u]Maria[/name_u]
[name_f]Iris[/name_f] [name_f]Charlotte[/name_f] [name_u]Maria[/name_u]
Which one would you pick right now?
Nell is my favorite!
Nell [name_f]Josephine[/name_f] [name_u]Maria[/name_u] would be a perfect sister for [name_f]Thea[/name_f].
Nell [name_f]Josephine[/name_f] [name_u]Maria[/name_u] 
Love all your options! [name_f]Thea[/name_f] and [name_f]Nell[/name_f] sound best together, imo.
Nell [name_f]Josephine[/name_f] really seems perfect! Another absolutely stunning, sweet, vintage, nickname name with the ability to sound grown up - I really love it!
Nell [name_f]Josephine[/name_f] for sure!
Nell [name_f]Josephine[/name_f] [name_u]Maria[/name_u] is my favorite!
Iris [name_f]Charlotte[/name_f] [name_u]Maria[/name_u] 
I think [name_f]Iris[/name_f] fits the best with [name_f]Thea[/name_f]!
[name_f]Nell[/name_f] and [name_f]Juno[/name_f] are good options too
Nell and [name_f]Thea[/name_f] are lovely! I also like [name_f]Juno[/name_f].
Nell is my favourite!! [name_f]Nell[/name_f] and [name_f]Thea[/name_f] sound like characters in a book, perfection.
Nell is my favorite. How such a short name can pack so much punch! I love [name_f]Iris[/name_f] too and [name_f]Juno[/name_f] is OK though seems like it is of a particular moment.
Oh, such a tough choice! They are all so sweet! But I have to give it to [name_f]Nell[/name_f]. I just love the vintage vibes of [name_f]Thea[/name_f] & [name_f]Nell[/name_f] together. <3
Nell [name_f]Josephine[/name_f] [name_u]Maria[/name_u]! [name_f]Thea[/name_f] & [name_f]Nell[/name_f] go perfect together! 
Iris [name_f]Charlotte[/name_f] [name_u]Maria[/name_u]
All of the three combos are beautiful! I think [name_f]Thea[/name_f] and [name_f]Nell[/name_f] would make a perfect sibset. Both have an old, sweet feeling.
Nell
If you had asked for suggestions I would have suggested these exact names, they fit beautifully with [name_f]Thea[/name_f]. [name_f]My[/name_f] favorite is [name_f]Nell[/name_f] [name_f]Josephine[/name_f] [name_u]Maria[/name_u] - it’s uncommon, feminine, and strong.
Oh my goodness, you’re killing me! I love all three beyond words. Keeping in mind big sister [name_f]Thea[/name_f], I’d pick [name_f]Juno[/name_f] [name_f]Elisabeth[/name_f] [name_u]Maria[/name_u].