His brother’s name is [name_m]Jasper[/name_m] and we have a 3 syllable Germanic surname. Here is our whittled down list:
[name_m]Aldo[/name_m]
[name_m]Axel[/name_m]
[name_u]Emmett[/name_u]
[name_m]Ernst[/name_m]
[name_m]Oskar[/name_m]
[name_m]Otto[/name_m]
Please vote! 
My favourites with [name_m]Jasper[/name_m] are
[name_m]Jasper[/name_m] and [name_m]Ernst[/name_m]
[name_m]Jasper[/name_m] and [name_m]Otto[/name_m]
[name_m]Jasper[/name_m] and [name_m]Axel[/name_m]
I like [name_m]Jasper[/name_m] and [name_m]Otto[/name_m], but [name_m]Ernst[/name_m] feels less predictable to me.
I like [name_m]Oskar[/name_m] and [name_m]Jasper[/name_m].
I like [name_m]Jasper[/name_m] and [name_m]Oskar[/name_m]. [name_m]Otto[/name_m] is also quite sweet.
I view [name_m]Jasper[/name_m] as rather modern but with history, the other three are either very old ([name_m]Ernst[/name_m]), dated ([name_m]Axel[/name_m]), of a very differnt style ([name_m]Aldo[/name_m], which feels very alternative to me) or too Twilight-y ([name_u]Emmett[/name_u]).
I like [name_m]Axel[/name_m] or [name_m]Otto[/name_m] with [name_m]Jasper[/name_m]
[name_m]Otto[/name_m] would be my first choice with [name_m]Jasper[/name_m], followed by [name_m]Ernst[/name_m].
I love [name_m]Jasper[/name_m] and [name_m]Ernst[/name_m], [name_m]Jasper[/name_m] and [name_m]Oskar[/name_m], and [name_m]Jasper[/name_m] and [name_m]Otto[/name_m]. I’m leaning toward [name_m]Otto[/name_m], but I think [name_m]Jasper[/name_m] and [name_m]Ernst[/name_m] are perfect together, and I just love [name_m]Oskar[/name_m] in general! I’m particularly fond of the [name_m]German[/name_m] spelling. 
Good luck!
My first thought is that [name_m]Jasper[/name_m] and [name_u]Emmett[/name_u] fit perfectly together, however, both are Twilight characters, so if it bothers you for people to make that connection than you shouldn’t use it.
I would say that [name_m]Jasper[/name_m] and [name_m]Axel[/name_m] would make a fine pairing.
I say [name_m]Axel[/name_m], as this is what we’ve just named our 11 week old son. 
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I like [name_m]Otto[/name_m] and [name_m]Oskar[/name_m] too. Another [name_m]German[/name_m] name I like is [name_m]Adler[/name_m].
[name_m]Aldo[/name_m] is a Scottish chavvy nickname for someone called [name_m]Alan[/name_m].
I like [name_m]Jasper[/name_m] and [name_m]Oskar[/name_m] best, with [name_m]Jasper[/name_m] and [name_m]Axel[/name_m] a distant second choice.
I like [name_m]Jasper[/name_m] and [name_u]Emmett[/name_u] together.
[name_f]Susan[/name_f]
I like [name_m]Jasper[/name_m] and [name_m]Otto[/name_m]!
[name_m]Jasper[/name_m] and [name_u]Emmett[/name_u] also sound good together, but they’re both Twilight characters, as others have pointed out, so you choose [name_u]Emmett[/name_u], know that some people will make that connection.
[name_m]Jasper[/name_m] and [name_u]Emmett[/name_u]! They both sound like Brothers!
I love [name_m]Oskar[/name_m], [name_m]Axel[/name_m] and [name_m]Otto[/name_m] from your list. I think [name_m]Jasper[/name_m] and [name_m]Otto[/name_m] sound best together, wonderfully quirky!
I think [name_m]Ernst[/name_m] & [name_m]Jasper[/name_m] go together really well! Very cute sibling set. I also like [name_m]Oskar[/name_m] and [name_m]Otto[/name_m] from your list.
[name_m]Oskar[/name_m] 
[name_m]Jasper[/name_m] and [name_m]Ernst[/name_m] sounds fantastic
I have a soft spot for [name_m]Otto[/name_m] <3 It’s dignified but endearing, and ends in that irresistible, jaunty O. It goes great with Jasper too, but that shouldn’t be the main focus.
All 6 are lovely in their own way and any of them could work beautifully with [name_m]Jasper[/name_m]! Here are my thoughts on each:
[name_m]Aldo[/name_m] - a great Name for any age! Works well for someone with a strong, “Blokey”, “Outdoorsy” personality. I love the sweet Nickname Aldi but that’s been ruined by the worldwide Chain Store, unfortunately!
[name_m]Axel[/name_m] - Cheeky and bright and fabulous!
[name_u]Emmett[/name_u] - This is the only Name that I have never heard of as a [name_m]German[/name_m] Name before but it probably works the best with [name_m]Jasper[/name_m]!
[name_m]Ernst[/name_m] - I [name_u]LOVE[/name_u] [name_m]Ernst[/name_m] as we have always joked that this would have been our Daughter’s Name if she’d been a Boy because she is SO [name_m]EARNEST[/name_m]! For me [name_m]Ernst[/name_m] could only work for an earnest “Old Soul” [name_u]Baby[/name_u] who spends his time contemplating the ways of the world (just like our Daughter did and STILL DOES eleven years later!)! For some reason I can only imagine it as a Name for a [name_f]Strawberry[/name_f] Blonde or [name_u]Red[/name_u] Head [name_u]Baby[/name_u] Boy!
[name_m]Oskar[/name_m] - [name_f]Lovely[/name_f] Name which works well with a wide variety of personalities, looks and ages, but might be a little too similar to [name_m]Jasper[/name_m], but then again that might make it the perfect choice!
[name_m]Otto[/name_m] - Sweet, chubby, serious little [name_u]Baby[/name_u]!
All the very best deciding and with your delivery!