[name_f]Penelope[/name_f] is my fave for a girl with those sisters. They give me a very [name_m]British[/name_m] vibe together. Others that spring to mind:
[name_f]Imogen[/name_f]
[name_f]Matilda[/name_f]
[name_f]Florence[/name_f]
[name_f]Cecilia[/name_f] or [name_f]Cecily[/name_f]
[name_f]Evelyn[/name_f]
[name_f]Romilly[/name_f]
[name_f]Josephine[/name_f]
For a boy, loving [name_m]Alistair[/name_m] or [name_m]Louis[/name_m] especially. Also [name_m]Jasper[/name_m] if you don’t mind the same starting sound. I also think of:
[name_m]Arthur[/name_m]
[name_m]Theodore[/name_m]
[name_m]Rupert[/name_m]
[name_m]Benjamin[/name_m]
[name_m]Samuel[/name_m]
[name_m]Frederick[/name_m]
[name_m]Oliver[/name_m]
[name_m]Alfred[/name_m]
[name_m]Sebastian[/name_m]