How bout:
[name_f]Sonnet[/name_f] nn Sonny/ [name_f]Sunny[/name_f], musical but links with brother name via the nickname…
Duet… I’ve always loved this but usually in the middle but could make an unusual first
[name_m]Aalto[/name_m]: sounds musical means wave… i think
[name_f]Vita[/name_f]: means life and sounds so lively
[name_f]Blythe[/name_f]: maybe to vintage for you but such a happy meaning and 1 syllable
[name_f]Dulcie[/name_f]: sweet meaning and musical… maybe too vintage but i think could have a revival
[name_m]Jazz[/name_m]: cool, relaxed and fun
All the best!!