I think [name_f]Ivy[/name_f] [name_f]Pearl[/name_f] is the perfect sister.
[name_m]Thomas[/name_m] [name_m]Patrick[/name_m] goes nicely with [name_f]Rose[/name_f]
Other suggestions:
[name_m]Theodore[/name_m] ([name_u]Theo[/name_u])
[name_m]Bruno[/name_m]
[name_m]Heath[/name_m]
[name_m]Henry[/name_m]
[name_m]Charles[/name_m]
[name_u]Jude[/name_u]
I also think [name_f]Ivy[/name_f] [name_f]Pearl[/name_f] works best. [name_f]Ivy[/name_f] and [name_f]Rose[/name_f] would make great sisters. As for boys, I really like both [name_m]Axel[/name_m] [name_m]Thomas[/name_m] and [name_m]Thomas[/name_m] [name_m]Axel[/name_m]. I also prefer [name_f]Ivy[/name_f] because you’re considering [name_m]Axel[/name_m], since [name_f]Hazel[/name_f] and [name_m]Axel[/name_m] are very similar to each other.