What lovely names! :D I would use
Margaret Pearl NN
Maisie or
Greta, depending on what she ends up looking like or acting like. The nice thing about
Margaret is the number of possible NNs - if
Maisie or
Greta don't seem right, there's always
Maggie,
Margie,
Daisy, and I'm sure you could come up with a few more if you needed to.
The best fitting siblings would be
Rosemary NN
Romy and
Lillian (possible NN Lily/Lilly?)
Elisabeth NN
Elsie is a close choice, but
Elsie is too close to
Maisie, IMO.