I would consider a more formal name for [name_f]Lucy[/name_f], but still use the nickname. A name that better matches sister Katherine’s name.
Perhaps….
Sim0n C@rl, K@therine (Kate) [name_f]Lily[/name_f], M@rk Juli@n and….
[name_f]Louisa[/name_f] [name_f]Margaret[/name_f] (Lucy)…Simon, [name_f]Kate[/name_f], [name_m]Mark[/name_m] and [name_f]Lucy[/name_f]
Other ideas:
[name_f]Margaret[/name_f] Genevieve…Simon, [name_f]Kate[/name_f], [name_m]Mark[/name_m] and [name_f]Meg[/name_f], [name_f]Margot[/name_f], [name_f]Maggie[/name_f], [name_f]Marge[/name_f], [name_f]Greta[/name_f], [name_f]Mae[/name_f], [name_f]Maisie[/name_f], [name_f]Marit[/name_f]
[name_f]Marjorie[/name_f] (Margery) [name_f]Eugenia[/name_f] (Margie)…Simon, [name_f]Kate[/name_f], [name_m]Mark[/name_m] and [name_f]Margie[/name_f]
[name_f]Margaret[/name_f] [name_f]Eugenia[/name_f] (Greta)…Simon, [name_f]Kate[/name_f], [name_m]Mark[/name_m] and [name_f]Greta[/name_f]
[name_f]Margot[/name_f] Genevieve…Simon, [name_f]Kate[/name_f], [name_m]Mark[/name_m] and [name_f]Margot[/name_f]
[name_f]Eugenia[/name_f] [name_f]Margaret[/name_f] (Gina,Genie)…Simon, Kate,Mark and [name_f]Genia[/name_f] or [name_u]Genie[/name_u]
[name_f]Genevieve[/name_f] [name_f]Margaret[/name_f] (Gen, Evie)…Simon, [name_f]Kate[/name_f], [name_m]Mark[/name_m] & [name_f]Evie[/name_f] or Gen(ny)
You might consider these forms of [name_f]Margaret[/name_f], because they are more similar to Katherine’s middle (Lily.)
[name_f]Louisa[/name_f] [name_f]Mae[/name_f]
[name_f]Genevieve[/name_f] [name_f]Maisie[/name_f]
[name_f]Eugenia[/name_f] [name_f]Mari[/name_f]
After looking over all these suggestions, my first choice with the sibling names is [name_f]Louisa[/name_f] [name_f]Margaret[/name_f], nicknamed [name_f]Lucy[/name_f].
Sim0n, K@te, M@rk and [name_f]Lucy[/name_f]
Second choice would be [name_f]Genevieve[/name_f] [name_f]Mae[/name_f] (or some name to honor Margaret) nicknaming her [name_f]Genny[/name_f] or [name_f]Evie[/name_f].