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Katherine & Kate are very timeless. I don't think there will be a problem with them ever feeling dated. Personally, I would prefer the longer version as the given name, with Kate as the nickname. That'll give her more options if she decides at some point that she wants them.
I find Cece for Lucille to be a bit of a stretch, but not totally inplausible either.
I do think Lucille called Cece is different enough. However, even if it wasn't, I would still encourage you to use Lucille if it's the name you love best. If your friend were currently pregnant with a girl and had claimed Lucy, I might feel differently. But she's not currently pregnant, may never have a girl, and could possibly no longer even like Lucy if she does have a girl one day. I'm generally not on board with the idea that people can call "dibs" on a name for perpetuity.
It's also possible that your daughter will not like Cece and will prefer Lucy as her nickname. Worst case scenario is that one day your friend does have a Lucy and you have to use the full Lucille when the girls are together. Hardly the end of the world.
I really like Valerie Simone.
I'd say what your father and his family are doing is emotional blackmail. They may not like the relationship you have with your step-mother, but they should accept that she is an important part of your life.
I'd say that Irene & Olga are not ready for a comeback for most people, but I can see both names working in certain pockets of the U.S. I particularly like Olga and would be tempted to use it as a first name, both ecause I like it & also to make a statement to your father's side of the family. But that sounds more confrontational than you want to be.
You could also consider using their middle names, if those are more to your style. Or variants of Irene like Irina, Iria or Arina. I also like the suggestion of using names with similar meanings. Though there aren't really as many workable variants of Olga. You could also consider using their initials, which would open you up to any I & O names that you like.
I don't see any reason why you shouldn't use Sofie instead of Sophie. There will be a need for clarification, but I hardly think explaining "Sofie with an F" is going to be a huge issue.
I wouldn't exactly call Stacy classic. For a name to feel classic for me, it needs to have had at least some usage for several generations. Stacy as a given name had a very short spike that gives it a formerly-trendy and currently-dated feel.
However, I don't think you should eliminate it simply because it doesn't feel as classic as the other names you like. Names can have other ways of feeling similar. For example, Stacy & Sophie both start with S, end in "ee" and have a youthful/spunky feel to me. Perhaps you'd also like other names with some or all of these qualities.
Is there something specific you are looking for? Or just people's general impressions?
I like Leona and think it'd fit well with other more common names like Eleanor/Lenora. I could see it striking some people as perhaps a bit dated, but I also think it may be on the cusp of becoming at least slightly more popular. It entered the top 1000 in 2009 and is currently ranked 738. If you are looking for something familiar but on the rare side, Leona would be a very good pick.
I'm not really a fan of boy's names on girls, so no, I don't particularly care for Shayne for a girl.
I also agree with previous comments that it seems a bit dated, and I'm not crazy about the Y being thrown in. If someone was going to use it for a girl, I'd prefer keeping the traditional spelling of Shane.
I didn't see the pattern until it was pointed out. Honestly, even after it was pointed out, I don't think it really is a pattern because Isla's name doesn't fit.
Perhaps if you explained to your son that you don't want to do another boy name that fits because you don't want her to feel like her name is left out? Another option could be to point out that both boy names have E's in them & you'd rather follow that pattern instead? In that case, I Theo could work (it's also my favorite from your list).
From your girl list, I really like Carys & Cecily. I agree with previous comments that Lola seems too similar to Isla.
I really like James & Jeremy from your list. I also really like the suggestions of Jude & Jasper.
I'll suggest (or 2nd) Julian & Joseph.
They are only 2nd cousins, won't be seeing it each other often, (I assume) have different last names, plus one is a boy & the other a girl. Plus, you & your cousin have equal claim to the grandmother. I think it's fine.
I remember in your previous posts that you always talked about Ava Louise. I don't think you ever considered another middle name for her. Could part of the problem be that you've always thought of her as "Ava Louise" and never "just Ava?" I think Ava Louise as a double barrel is lovely. Perhaps you'd feel better about calling her by both names all the time?
But I also think you shouldn't rule out hormones combined with exhaustion. Or the fact that it is sometimes easier to worry about a name as opposed to more serious issues like actually taking care of the baby & being a good mother (things we all worry about sometimes).
When my youngest was born, I was so excited to get to use the name I loved but had never suggested out loud because I was sure my husband wouldn't like it. Then, after he was named, it was like I kept forgetting what to call him. I knew it started with a G, but I was certain we hadn't really named him George, and I kept calling him by other G names. I'd sometimes go through 2 or 3 before I'd land on George, and then I'd struggle because it just didn't seem quite right. This lasted 2 or 3 weeks and then I slowly eased back into loving (and remembering) his name. It was also around this time that I think my hormones regulated & I started getting a bit more sleep. If calling her the full Ava Louise doesn't help, perhaps you just need to give it another week or two. I suspect her name will start to feel more right once you're feeling better and have started to settle into a routine.
I like Ulysses and think it would likely do fine in some areas. In others, it might get a few comments or raised eyebrows, but I don't think it would be considered awful so much as unusual.
If the original Grant in your family was named after the general, then I think Ulysses works as an honor name. In fact, I think it'd be a fun thing to do, similar to picking an honor name because of a shared meaning or initial. However, I'd want to be certain that the original Grant really was named after the general. If not, then I think the connection could be stretching things a bit.
I agree with previous comments that your husband's side (especially if Leszek is his paternal grandfather) is already being honored by the use of the his surname. I also agree that Thorvald is easier for American English speakers to say/spell. With a less common first name and a last name that is likely difficult, I think an easier middle could be a plus in the long run. Bonus points also awarded because Thorvald is pretty awesome.
Oh, good point! I didn't notice it until it was mentioned, but now that I have, I can't seem to get over the association. I think I shall change my vote to Arthur.
Oh this is hard, as I really like both Walter & Arthur. I think I lean more towards Walter, mostly because I would prefer Art/Artie as a nickname for Arthur.
Good point, Dorothy probably is technically for a comeback for most people. I should clarify that my mother's name is Dorothy, nn Dot, so it doesn't feel ready for me. FWIW, she has always hated her "old lady" name.
I really like Avis & Vesper and I don't think they sound unisex at all. I'm surprised at the problems with your family, Avis & Vesper both have latin roots, so I would think most Italian or Spanish speakers would get them, even if they weren't personally familiar with either name.
Dorothy is nice, but it does strike me as a huge departure from your usual style. That's OK, but it should be something to consider if you plan on more kids and prefer for sibling names to coordinate. Would you feel locked in to using only names that "match" Dorothy?
An alternative might be Dorothea, which feels a little fresher to me.
I really like it & I think it sounds fine with the surname.
Of the 3 options, I like Charlie or Leo best.
If Charlie is a nickname & the actual birth certificate name will be Charles, then Charles/Charlie would get my vote over Leo.