I’m mom to a [name_f]Viola[/name_f] as well, although we pronounce her name Vie-OH-la. Anyway, we sure have great taste! 
I think [name_m]Sebastian[/name_m] is the perfect match for [name_f]Viola[/name_f]'s brother.
Although I love [name_m]Wilder[/name_m], I can’t help but find it a bit trendy in comparison to your daughter’s name. Also, as you have said, with a one-syllable last name, it’s best to go a bit longer .
Suggestions: [name_m]Alexander[/name_m], [name_m]Lysander[/name_m], [name_m]Fabian[/name_m], [name_m]Adrian[/name_m], [name_m]Julian[/name_m], [name_m]Benjamin[/name_m], [name_m]Phineas[/name_m], [name_m]Joachim[/name_m], [name_m]Ulysses[/name_m] and [name_m]Olivier[/name_m].
Less than two syllables: [name_m]Jasper[/name_m], [name_m]Caspar[/name_m], [name_m]Florin[/name_m], [name_u]Tobin[/name_u], [name_m]Desmond[/name_m], [name_m]Ambrose[/name_m], [name_m]Amos[/name_m], [name_m]Leon[/name_m], [name_m]Arthur[/name_m], [name_m]Florin[/name_m], [name_u]Ellis[/name_u], [name_m]Louis[/name_m], [name_m]Abram[/name_m], [name_m]Edgar[/name_m], [name_u]Wallace[/name_u], [name_m]Winston[/name_m], [name_m]Albert[/name_m], [name_m]Lucas[/name_m] and [name_m]Hugo[/name_m].