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I vote for Abigail and Isabelle as well, but would you consider spelling it Isabel? I think it looks better paired with Abigail since they would both end with an "L" but it's just a personal preference and I realize not the traditional French spelling like you have.
I love your other combos as well, and would be delighted to hear twins named with any of these.
I would either use as middle names or alter the names slightly to something like Mara and Jana. It's not worth the lifetime of 420 jokes.
You already have a lot of great suggestions including that it's perfectly fine to not use and M name. But I wanted to jump in with how much I LOVE Maxwell and would use it with the nn Wells so it's the opposite of what others are saying - M name with non-M nn.
I like Miles or Morris as other boy M names. I also love Clemantine for a girl!
Some more ideas, some more international than others:
August, Winston, Mayer, Bennett, Charles, Elliott, Luca, Lachlan, Max, Nolan, George, Ezekial, Calvin, Franklin, Arlo, Milo, Theodore
Your style is very similar to mine (though I love NNs), so here are a few others I love:
Sabine, Sadie, Leona, Lena, Cecily, Cecilia, Siena, Rose, Hadley, Violet, Miranda, Marissa, Poppy, Naomi, Esther, Anna
FWIW I love the name Elena and have seen it used across many cultures. I much prefer that spelling to Elaina. Alana and Alaina are other variations you might like.
Maverick is another Teen Mom OG baby name. He's about 2 years old now, so that's why it spiked in 2017.
My dad was supposed to be named A.B. but when it came time to do the birth certificate my grandfather put down B.A. and as you can imagine my grandmother was pissed. So most of his childhood he went by A, which is what his mother and extended family called him. But my grandfather called him B, and as he got older he chose to be called by his first name. He still has some extended family that calls him by his middle name. I don't think it ever bothered him that much. It confused me as a kid when we went to family reunions, but once he explained it wasn't a big deal.
I prefer Celine as the first name, with that spelling.
I like Juniper Louise/a or Louisa June are great options! I would take those as my top 2.
I also like Georgia June or Georgia Louise.
Are both grandmas and MIL still with us? That might also affect my preference. Who did you honor with your sons' names? And are June/Louise on your side or your H's?
I think you are solid on boy names - it's just a matter of choosing. Girl names I agree with some of the others on. August is all boy. Auden is heavily boy to me, but works better for a girl than August since it's less common. Anne is lovely, but maybe not your style. Avery fits as a truly gender neutral option, but sounds like that's out. Here are a few other ideas:
June, Juniper, Summer, Meredith, Julianne, Juliet, Julene, Liana, Willa, Willow, Wilhelmina, Harlow, Haven, Vera, Vada, Olive
My favorites from your list are Almila, Arin, Ayla, Mavera, Patya, Yiva and Maysa.
I'm going to ignore the middle options for now and focus on the first names. IMO, the ones that work best in an English-domininated culture are: Ayaz, Balamir, Erdenay, Erem, Ibrahim, Omur, Rua and Talu
Every time I see Fatih I read it as Faith misspelled, though it's not my first time seeing the name. Kayra I think most English speakers would assume is a girl due to the similarities with Kara and Kayla, but obviously that would change upon meeting him.
Iris is a great choice! Iris Juniper is a little too botanical for my taste, but if you love it use it. What about Iris Mae? Iris Eloise? Iris Miranda? Iris Juliet? Iris Blake?
I like Samuel, Nathaniel, Daniel or Dashiell, which pull in the L sound in both of your girls' names.
A few other thoughts... William, Henry, Theodore, Zachary, Tristan, Arthur, Miles, Felix
I would avoid it. We have a few Alexa devices in our home and I say her name at least a dozen times a day. The number of products that integrate with it is growing rapidly. I don't see it going away in the next decade, at least. Alexa and Alexis are both among the top 10 fastest falling names in 2016, I would assume because of the device. One of my friends is named Alisa and it drives her nuts - her husband calls her Alexa by accident all the time, and other times he calls her by her name but Alexa responds.
I really like Luna Lorien Wilder. If that's your favorite name and DH is on board, I don't see any need to keep looking.
I prefer June to Simone, but I LOVE Gloria. Go with Gloria June!
I like all of your options except Camden, which is all New Jersey to me. The only other one that made me do a double take was Cameron - my mind instantly went to John Cameron (the movie producer), which isn't a bad thing but the name feels backwards to me because of the association.
Going on style alone, I like Wren best. Millicent seems mismatched with Grayson for me, but I do love Milly. Addelyn I prefer to be spelled Adelyn, but that's a nice option.
Have you considered Virginia Rose or Virginia Noelle? Those are my favorite options for you.
I really like Oscar. I know a five-year-old named Oskar and I just love it on him. Perhaps you would also like Otto?
Walter is a bit too old man for my taste, but Breaking Bad has freshened it up a bit. I wouldn't think twice about siblings Silas and Walther.
I prefer non-matching initials, and Sil and Sol are very similar, but style-wise I think Silas and Solomon go well.
A few other ideas...Adrian, Gabriel, Ronan, Abner, Nolan, Edwin, Tristan, Quincy, Jeremiah, Dorian, Vincent
I prefer either Oliver or Vɑlɛntin. Lɛvi is a bit too close to Lucɑ for my taste, and I would still assume a boy named Lɛvi is Jewish. Oliver is a more popular choice, and Vɑlɛntin matches the style of Lucɑ nicely.
If you are willing to share, I'm curious about the circumstances since it sounds like your son has already been born. Does he have a name and you want to change it? Or are you adopting? Or is your son not born yet and you are making assumptions about him based on his brother?