As a sibling for [name_u]Jameson[/name_u] (nn [name_u]Jem[/name_u]). A brother and a sister- any name at all- what do you think matches it best?
[name_u]Jameson[/name_u] sounds great with:
[name_m]Theodore[/name_m] (nn [name_u]Teddy[/name_u])
[name_m]Frederick[/name_m] ([name_m]Fred[/name_m])
[name_m]Walter[/name_m]
[name_m]Harold[/name_m] (nn [name_m]Hank[/name_m])
[name_m]Felix[/name_m]
[name_m]Ambrose[/name_m]
[name_m]Roger[/name_m]
[name_u]Curtis[/name_u] (nn [name_m]Curt[/name_m])
[name_f]Katrina[/name_f] (nn [name_f]Kat[/name_f])
[name_f]Cora[/name_f]
[name_f]Lucy[/name_f]
[name_f]Sybil[/name_f] (nn Sybbie)
[name_f]Daphne[/name_f]
[name_f]Sarah[/name_f]
[name_f]Lily[/name_f]
[name_f]Caroline[/name_f] (nn [name_f]Cara[/name_f])
[name_f]Linnea[/name_f]
[name_f]Miranda[/name_f]
Girl:
Ketherine nn [name_m]Kitt[/name_m]
[name_f]Nathalie[/name_f]
[name_f]Rosalie[/name_f]
[name_f]Juliette[/name_f] / [name_f]Juliana[/name_f] nn Jools
[name_f]Morgana[/name_f] nn [name_u]Morgan[/name_u]
Any clasic girl name with a modern nn would fit.
Boy’s,
[name_m]Balthazar[/name_m]
[name_m]Sebastian[/name_m]
[name_m]Victor[/name_m] nn Vik / vic
[name_m]Bennet[/name_m]
[name_m]Becket[/name_m]
You can go with any clasic boy’s name and too spice up the sibset you could go with a shakesperian name… [name_u]Jameson[/name_u] will fit with any clasic name. If you wanna go for a theme in the sibset you could go with names like [name_u]Walker[/name_u] and [name_f]Brandi[/name_f]… u get it. 
I think [name_m]Graham[/name_m] would match well for a boy - while [name_f]Whitley[/name_f] or [name_u]Whitney[/name_u] would be a lovely name for a sister.
For [name_u]Jameson[/name_u]'s sister, I would choose…
[name_u]Meredith[/name_u]
[name_f]Constance[/name_f]
[name_f]Susannah[/name_f]
[name_f]Beatrice[/name_f]
[name_f]Henrietta[/name_f]
[name_f]Winifred[/name_f]
[name_f]Victoria[/name_f]
[name_f]Louisa[/name_f]
[name_f]Virginia[/name_f]
[name_f]Theodora[/name_f]
[name_f]Matilda[/name_f]
[name_f]Eleanor[/name_f]
[name_f]Harriet[/name_f]
[name_f]Augusta[/name_f]
[name_f]Alice[/name_f]
For [name_u]Jameson[/name_u]'s brother, I would pick…
[name_m]Maxwell[/name_m]
[name_u]Spencer[/name_u]
[name_m]Randolph[/name_m]
[name_u]Porter[/name_u]
[name_m]Philip[/name_m]
[name_u]Douglas[/name_u]
[name_m]Percival[/name_m]
[name_u]Emmett[/name_u]
[name_m]Montgomery[/name_m]