I really love both, but I think [name_f]Heidi[/name_f] [name_m]Joann[/name_m] is my favourite with [name_f]Bridget[/name_f] and [name_m]Grant[/name_m]! [name_f]Bridget[/name_f], [name_m]Grant[/name_m], and [name_f]Heidi[/name_f] feel so cohesive without being matchy, familiar, cheerful, and a little spunky. [name_f]Heidi[/name_f] [name_m]Joann[/name_m] also has such a sweet, vintage-but-not-fussy feel.
That said, I completely understand why you’re torn over [name_f]Sylvie[/name_f]. I think [name_f]Sylvie[/name_f] is gorgeous and has that same bright, lively quality. It does feel more on-trend to me than [name_f]Heidi[/name_f], though, whereas [name_f]Heidi[/name_f] feels a little more unexpected right now. If “not heard everywhere” is a big priority, I’d probably lean [name_f]Heidi[/name_f].
A few others that came to mind for your style:
[name_f]Lucy[/name_f]
[name_f]Greta[/name_f]
[name_f]Ruby[/name_f]
[name_f]Della[/name_f]
[name_f]Molly[/name_f]
[name_f]Tessa[/name_f]
[name_f]Nina[/name_f]
[name_f]Pippa[/name_f]
[name_f]Cleo[/name_f]
[name_f]Lydia[/name_f]
[name_f]Georgia[/name_f]
But honestly, [name_f]Bridget[/name_f], [name_m]Grant[/name_m] & [name_f]Heidi[/name_f] is such a great sibling set. I’d probably choose [name_f]Heidi[/name_f]!