[name_m]Reuben[/name_m], [name_m]Bryan[/name_m] or [name_u]Fionn[/name_u] ([name_u]Fee[/name_u]-on) as a little brother for [name_f]Ursula[/name_f] and [name_f]Gwendoline[/name_f] ([name_f]Gwen[/name_f])?
I think [name_u]Fionn[/name_u] goes perfectly with them! I think [name_m]Reuben[/name_m] could also work with them! [name_m]Bryan[/name_m] is my personal favorite but feels a little too popular with the other names!
Fiona goes best with your girl names, I think.
Fionn gets my vote! It’s a great name and works well with the sibset
Reuben!! It has the same whimsical feel. I also like [name_u]Fionn[/name_u], but strongly dislike [name_m]Bryan[/name_m] - it sounds clunky + dated next to his sisters.
My favourite is definitely [name_u]Fionn[/name_u] (although I would pronounce it the same as [name_u]Finn[/name_u]). I had it in my UC for a while - I adore it!
I like [name_u]Fionn[/name_u]!
Reuben!
I adore [name_m]Reuben[/name_m] but [name_u]Fionn[/name_u] goes better in this sibset ! 
Fionn
Fionn and [name_m]Reuben[/name_m] get my vote!
Fionn goes really well
Fionn!! so handsome and sweet, and fits in nicely with U & G! [name_m]Reuben[/name_m] would be my second choice! [name_m]Bryan[/name_m] seems a bit plain and expected with your girls names, in my opinion.
good luck! 
Fionn is amazing!
Rueben or [name_u]Fionn[/name_u]
Reuben ! And I think [name_u]Fionn[/name_u] would be a great pair for a middle !
I was originally thinking [name_u]Fionn[/name_u] worked better with [name_f]Ursula[/name_f] and [name_f]Gwendoline[/name_f], but now I’m not sure? [name_m]Reuben[/name_m]’s by far my favorite of the three options, but I had been willing to choose [name_u]Fionn[/name_u] over it because it worked so well with [name_f]Ursula[/name_f] and [name_f]Gwendoline[/name_f]. But after seeing [name_f]Ursula[/name_f], [name_f]Gwendoline[/name_f], and [name_m]Reuben[/name_m] written out, I’m not too sure! I think all three work really well together. So I’m voting with my heart and going [name_m]Reuben[/name_m].
Good luck!