What a beautiful, Georgian/Regency name!
For her sibling, how about:
Frances (Ida & Franny!)
Coral
Polly
Cecily
Bronte
Harriet
Romilly
Hester
Xanthe
Camille
Elspeth
Bryony
Saskia
Zélie
Kitty
I think Ambrose or Conrad would be perfect, but otherwise:
Felix
Otis
Hugo
Anders
Jonty
Rupert
Wilfred
Edward
Barnaby
Jasper
Rafferty
Stanley
Swanson
Benedict
Aloysius (nn “Wish”)