We have about a week left to decide 
Their older brother is [name_m]John[/name_m].
[name_m]William[/name_m] and [name_f]Bronwen[/name_f]
or
[name_m]William[/name_m] and [name_u]Vivian[/name_u]
[name_m]William[/name_m] and [name_u]Vivian[/name_u]. [name_f]Bronwen[/name_f] is a drastic departure from the kings of the classics [name_m]John[/name_m] and [name_m]William[/name_m].
[name_m]William[/name_m] and [name_u]Vivian[/name_u] work perfectly together. I agree with the above poster that [name_f]Bronwen[/name_f] is a big departure from the classic naming style. [name_u]Vivian[/name_u] [name_f]Bronwen[/name_f] is nice, though.
I don’t like the “on” sound repeating in [name_f]Bronwen[/name_f] and [name_m]John[/name_m].
I love how the letters in [name_m]William[/name_m] and [name_u]Vivian[/name_u] share similarities without being overly close, and they sound really nice together. Stylistically, they match nicely, too.
[name_m]William[/name_m] & [name_u]Vivian[/name_u] gets my vote, hands down.
I like the pp suggestion of [name_u]Vivian[/name_u] [name_f]Bronwen[/name_f].
[name_u]Vivian[/name_u] gets my vote. I dislike [name_f]Bronwen[/name_f] and [name_m]John[/name_m] together.
[name_u]Vivian[/name_u] has nickname potential, as does [name_m]William[/name_m]. [name_f]Bronwen[/name_f] does not.
I like [name_m]John[/name_m], [name_m]William[/name_m] and [name_f]Bronwen[/name_f].
Of those options, [name_u]Vivian[/name_u] is most suited to brothers [name_m]John[/name_m] and [name_m]William[/name_m], because they are so, so classic - the kings, as another poster pointed out. [name_u]Vivian[/name_u] has a more glamorous vintage flair, but still fits with such classic names. [name_f]Bronwen[/name_f], on the other hand, is a noted departure.
I think a simple classic name is best-suited to the sister of [name_m]John[/name_m] and [name_m]William[/name_m], more classic even than [name_u]Vivian[/name_u], and perhaps more simple. Something like [name_f]Anna[/name_f], [name_f]Charlotte[/name_f], [name_f]Emma[/name_f], [name_f]Laura[/name_f], [name_f]Lucy[/name_f], [name_f]Mary[/name_f], or [name_f]Sarah[/name_f]. However, I think a more perky or slightly less used classic would be great, like [name_f]Clara[/name_f], [name_f]Eva[/name_f], [name_f]Elsa[/name_f], [name_f]Frances[/name_f], [name_f]Lena[/name_f], [name_f]Mabel[/name_f], [name_f]Nora[/name_f], [name_u]Ruby[/name_u], [name_f]Sophie[/name_f], or [name_f]Stella[/name_f].
My favorite is [name_f]Clara[/name_f], for sure! But, I also love [name_f]Charlotte[/name_f], [name_f]Sarah[/name_f], and [name_f]Stella[/name_f] in particular. [name_u]Vivian[/name_u] or [name_f]Bronwen[/name_f] would be lovely middle names.
[name_m]John[/name_m], [name_m]William[/name_m], and [name_u]Vivian[/name_u] fit nicely. [name_f]Bronwen[/name_f] seems a little out of place
[name_u]Vivian[/name_u] gets my vote 
[name_f]Bronwen[/name_f] is a lovely name too, though.
[name_m]John[/name_m], [name_m]William[/name_m] and [name_u]Vivian[/name_u].
Of the two, [name_u]Vivian[/name_u] gets my vote to go with [name_m]William[/name_m].
[name_u]Vivian[/name_u] ftw!
Double Congrats! I definitely choose [name_u]Vivian[/name_u] over [name_f]Bronwyn[/name_f]. [name_f]Bronwyn[/name_f] sounds too stuffy and dated to go with the classics of [name_m]John[/name_m] and [name_m]William[/name_m].
[name_m]William[/name_m] and [name_f]Bronwen[/name_f]. They are both very strong names.
[name_u]Vivian[/name_u] for sure. I agree with previous posters that [name_f]Bronwen[/name_f] seems like a stark departure in naming style from [name_m]John[/name_m] and [name_m]William[/name_m]. Plus I just like the name [name_u]Vivian[/name_u] more than the name [name_f]Bronwen[/name_f].