Penelope is wonderful and deserves something equally as polysyllabic and splendid. So, let's put Paisley in the mn position and give the girls reverse initials:
Penelope Ruth
Rosabel Paisley
Ruth and Rosabel have enormous 19th-century charm, and though Paisley is actually a very rare instance of a place name that I can enjoy on a person, it does clash with Penelope!
By the way, if they enjoy matchy names (initials or whatever), try them on Nel and Bel!