Woo - sentimental teenagers!
Honestly I think Rita is lovely. It has such a lovely old-Hollywood feel. What about Margarita? Then you sort of include Rita and Marie? If you wanted to honour her with Marie without using Marie, what about another one of Mary's variants? Maire, Mairi, Marion, Miriam, Mara.
I've always had a soft spot for Darla. I just think it's lovely (I also quite like Darling, but that's pretty clearly unusuable). What about Dahlia?
Zosia - ZO-sha, is the Polish form of Sophia (I feel like I'm always pushing Zosia onto people, I just want it to be used more), which *sort* of honours grandmother's Polish heritage and your other grandmother's middle name too.
I second Marguerite and Margot.
Also Daria and Dahlia.
Oh my, I love the suggestions of Pearl and Darling. I definitely think Eliza Darling and Josephine Pearl could be awesome sister names.