Is your family Finnish or Finnish-American?
I think Finnish names are fascinating. They always seem so light, yet strong. I think due to the extreme isolation of Finnish as a language it’s difficult to transfer them into a [name]Romance[/name] or Germanic naming context, but I wish they worked more easily.
Anyway, I want to throw a complete and total wrench in the works and say I don’t think any of your three choices are best for the first name slot.
I fear Otso, as a pp noted, will be teased as ‘fatso.’ The -o ending may be fashionable, but I can’t think of many English words, let alone names, that end with -so. It’s so unusual as a phoneme that I think it opens up your son to some very serious teasing (again, assuming you’re not raising him in Finland).
[name]Isidore[/name], to me, is the quintessential elderly Jewish man. The name means “gift of [name]Isis[/name]” but for some reason it took off like wildfire in the Sephardic Jewish community and now has a fusty and decidedly Semitic image (rather like [name]Milton[/name] or [name]Seymour[/name]).
[name]Hawthorne[/name] is very nice indeed. I like the ancient symbolism of the hawthorn tree and flower. But I don’t feel that it fits at all with brother [name]Uli[/name] and sister [name]Wren[/name], both of which are light, brisk names.
Suggestions: I know with [name]Miles[/name] and [name]Lyra[/name] you achieved a musical connection. Have you thought about [name]Arvo[/name]? Finnish composer [name]Arvo[/name] Part is one of my favorites, he did a lot of mind-bending stuff with classical music in the middle of the last century. I know [name]Arvo[/name] is certainly uncommon, but the sounds work well in English and in an English naming context.
Ismo is the Finnish form of [name]Ishmael[/name]. Again, I think the sounds work in an anglophone world, and your husband can get his nickname of [name]Izzy[/name].
[name]Eero[/name] = cool architectural connection; this is gaining some credence as a cool hipster name (though certainly very uncommon) even among people without any Finnish connection.
Jyri = my favorite Finnish name, and the local form of ‘[name]George[/name].’ This name just looks cool on the page. It works in English, though unfortunately I don’t have any musical or artistic associations.
[name]Timo[/name] = [name]Timothy[/name]. Easy to say and spell, looks masculine in the US. Would have a distinct sound from brother [name]Uli[/name]. There are multiple heavy metal musicians named [name]Timo[/name] (not sure if they’re the best namesakes, but still…)
I think you could move [name]Hawthorne[/name] to the middle spot. [name]Arvo[/name] [name]Hawthorne[/name] is a pretty cool name, imo.
If you want a jazzy middle to echo brother [name]Miles[/name]:
[name]Coltrane[/name]
Brubeck
[name]Thelonius[/name]
[name]Gillespie[/name]
[name]Calloway[/name]
[name]Ellington[/name]
[name]Duke[/name]
[name]Parker[/name]
[name]Armstrong[/name]
[name]Fitzgerald[/name]
Possible combos:
[name]Eero[/name] [name]Parker[/name]
Ismo [name]Gillespie[/name]
Jyri [name]Coltrane[/name]
[name]Timo[/name] [name]Calloway[/name]
etc…
[name]Glad[/name] to see that one considered.