SO MANY wonderful suggestions.
I would definitely use [name_f]Margaret[/name_f] ( or Margot) as a middle name and occasionally call her by the pet name [name_f]Margot[/name_f] at home so she know that it was a name you loved and had hoped to use. Perhaps at some point in her life she will decide to go by that name.
One suggestion stood out to me the name, [name_f]Amelie[/name_f]. I could imagine [name_f]Amelie[/name_f] [name_f]Margaret[/name_f], nicknamed [name_f]Amie[/name_f] (or [name_f]Mellie[/name_f] or Millie.)
Some other suggestions…
[name_f]Cassandra[/name_f] [name_f]Margaret[/name_f], nicknamed [name_f]Cassie[/name_f]
[name_f]Rowenna[/name_f] [name_f]Margaret[/name_f], nicknamed possibly [name_f]Winnie[/name_f]
Going with the idea of names similar to [name_f]Margot[/name_f]. I could imagine the following as sisters……
[name_f]Daphne[/name_f]
[name_f]Phoebe[/name_f]
[name_f]Audrey[/name_f]
[name_f]Eloise[/name_f]
[name_f]Fiona[/name_f]
[name_f]Claire[/name_f]
[name_f]Dorothy[/name_f], [name_f]Dorothea[/name_f]
[name_f]Eliza[/name_f], [name_f]Elise[/name_f]
[name_f]Penelope[/name_f]
[name_f]Ruth[/name_f]