You could stick with 4 ltrs, but with different initial and ending:
[name_m]Luke[/name_m]
[name_u]Rhys[/name_u]
[name_m]Jack[/name_m]
[name_m]Liam[/name_m]
[name_m]Joel[/name_m]
[name_m]Saul[/name_m]
I think something long would sound kind of out of place. I can’t imagine a sibset of [name_u]Evan[/name_u], [name_m]Otto[/name_m] and, say, [name_m]Sebastian[/name_m]. I really like the 4 letter connection. The first name that came to mind was [name_m]Alec[/name_m], for a boy. I think it fits really well with [name_u]Evan[/name_u] and [name_m]Otto[/name_m].
[name_u]Evan[/name_u], [name_m]Otto[/name_m] and [name_m]Miles[/name_m]
[name_u]Evan[/name_u], [name_m]Otto[/name_m] and [name_m]Peter[/name_m]
[name_u]Evan[/name_u], [name_m]Otto[/name_m] and [name_m]Linus[/name_m]
[name_u]Evan[/name_u], [name_m]Otto[/name_m] and [name_m]Rufus[/name_m]
[name_u]Evan[/name_u], [name_m]Otto[/name_m] and [name_u]Theo[/name_u]
[name_u]Evan[/name_u], [name_m]Otto[/name_m] and [name_u]Gus[/name_u]
[name_u]Evan[/name_u], [name_m]Otto[/name_m] and [name_u]Dexter[/name_u]
Girl
[name_u]Evan[/name_u], [name_m]Otto[/name_m] and [name_f]Lise[/name_f]
[name_u]Evan[/name_u], [name_m]Otto[/name_m] and [name_u]Arden[/name_u]
[name_u]Evan[/name_u], [name_m]Otto[/name_m] and [name_f]Margot[/name_f]
[name_u]Evan[/name_u], [name_m]Otto[/name_m] and [name_f]Ramona[/name_f]
[name_u]Evan[/name_u], [name_m]Otto[/name_m] and [name_f]Frances[/name_f]
[name_u]Evan[/name_u], [name_m]Otto[/name_m] and [name_f]Dahlia[/name_f]