Congratulations on your 2nd baby!
From your post, I understand that you are looking for familiar but not overused nature themed or meaningful names, perhaps some old-fashioned gems.
I adore all the feminine names on your list. [name_u]Celeste[/name_u] sounds amazing with [name_f]Sylvia[/name_f]! I really like the earth and sky subtle connection between the two names. [name_f]Noelle[/name_f] and [name_u]Esme[/name_u] are so sweet! [name_f]Rosie[/name_f] and all [name_u]Rose[/name_u] related names are breathtaking, in my opinion. @Kipperbo1 's previous suggestion of [name_f]Primrose[/name_f] is magnificent. [name_f]Elise[/name_f] is a beautiful classic. [name_f]Madeline[/name_f] is stunning too.
Some alternatives that you may also like are [name_f]Noelia[/name_f], [name_f]Neve[/name_f], [name_f]Celestia[/name_f], [name_f]Celia[/name_f], [name_f]Rosalie[/name_f], [name_f]Rosalind[/name_f], [name_f]Rosemary[/name_f], [name_f]Eliza[/name_f], [name_f]Eliana[/name_f], [name_f]Elodie[/name_f], [name_f]Emerald[/name_f], [name_f]Emmeline[/name_f], [name_f]Magda[/name_f], [name_f]Magdalena[/name_f] or [name_f]Magdalene[/name_f].
[name_u]Henry[/name_u] and [name_u]Elliott[/name_u] are great names for a boy. They’re charming, gentle, timeless and friendly sounding names.
Some of my first ideas for Sylvie’s sister were already mentioned by previous posters, but I would love to add some more options.
[name_u]Nature[/name_u] related with nature (flowers, plants, animals, water, astronomy, minerals, colors):
[name_u]Ember[/name_u]
[name_f]Avelina[/name_f]
[name_f]Stella[/name_f] or [name_f]Esther[/name_f]
[name_u]Dandelion[/name_u]
[name_f]Acacia[/name_f]
[name_u]Ruby[/name_u]
[name_f]Violet[/name_f]
[name_f]Gemma[/name_f]
[name_f]Aurora[/name_f]
[name_u]Seren[/name_u]
[name_f]Petal[/name_f]
[name_f]Dawn[/name_f]
[name_u]Holly[/name_u] or [name_u]Hollis[/name_u]
[name_f]Marina[/name_f]
[name_f]Cordelia[/name_f]
[name_f]Peony[/name_f]
[name_f]Ginger[/name_f]
[name_u]Lark[/name_u]
[name_f]Olivia[/name_f]
[name_f]Tulip[/name_f]
[name_u]Alba[/name_u]
[name_f]Summer[/name_f]
[name_f]Lilia[/name_f] or [name_u]Lilian[/name_u] or [name_f]Lillian[/name_f]
[name_f]Margot[/name_f]
[name_f]Celine[/name_f] or [name_f]Celina[/name_f] and [name_f]Selene[/name_f]
[name_f]Jasmine[/name_f]
[name_f]Philippa[/name_f]
[name_f]Meadow[/name_f]
[name_f]Augusta[/name_f]
[name_u]Laurel[/name_u]
[name_f]Tamara[/name_f]
[name_u]Clove[/name_u] or [name_f]Clover[/name_f]
[name_u]Hyacinth[/name_u] or [name_f]Jacinta[/name_f] or [name_f]Jacinda[/name_f]
[name_f]Linnea[/name_f]
[name_f]Maren[/name_f]
[name_f]Susannah[/name_f]
[name_f]Poppy[/name_f]
[name_u]Basil[/name_u]
[name_u]Sky[/name_u]
[name_m]Hawthorn[/name_m]
[name_u]Brook[/name_u]
[name_u]Forest[/name_u]
[name_u]Rain[/name_u]
[name_u]Murphy[/name_u]
[name_u]Leaf[/name_u] or [name_m]Leif[/name_m]
[name_u]Gale[/name_u]
[name_u]Augustine[/name_u]
[name_u]Callum[/name_u]
[name_u]Florian[/name_u]
[name_u]Indigo[/name_u]
[name_m]Cyrus[/name_m]
[name_u]Jupiter[/name_u]
[name_u]Linden[/name_u]
[name_m]Marshall[/name_m]
[name_u]Laurence[/name_u]
[name_u]Sterling[/name_u]
[name_u]Rhodes[/name_u]
[name_u]Lake[/name_u]
[name_u]Sage[/name_u]
[name_u]Arthur[/name_u]
[name_u]Garland[/name_u]
[name_m]Philip[/name_m]
[name_m]Cosmo[/name_m]
[name_u]Rye[/name_u]
[name_u]Thyme[/name_u]
[name_u]Quince[/name_u]
[name_m]Rafferty[/name_m]
[name_u]Ocean[/name_u]
[name_m]Moss[/name_m]
[name_m]Leonel[/name_m] or [name_m]Leonard[/name_m] or [name_m]Leonardo[/name_m]
[name_u]Aspen[/name_u]
[name_m]Clay[/name_m]
[name_u]Wren[/name_u]
[name_u]Ray[/name_u]
[name_m]Oliver[/name_m]
[name_m]Stellan[/name_m]
[name_u]Perry[/name_u]
Names with wonderful meanings:
[name_f]Belinda[/name_f]
[name_f]Theresa[/name_f]
[name_f]Anna[/name_f] or [name_f]Anita[/name_f] or [name_f]Annette[/name_f] or [name_f]Annabelle[/name_f]
[name_f]Cressida[/name_f]
[name_f]Adele[/name_f] or [name_f]Adelia[/name_f] or [name_f]Adeline[/name_f] or [name_f]Adelaide[/name_f]
[name_f]Evangeline[/name_f]
[name_f]Helena[/name_f]
[name_f]Joy[/name_f]
[name_u]Liberty[/name_u]
[name_f]Carys[/name_f] or [name_f]Cerys[/name_f]
[name_f]Alice[/name_f]
[name_f]Beatrice[/name_f] nn [name_f]Bee[/name_f] (to keep the nature connection)
[name_f]Diana[/name_f]
[name_u]Blythe[/name_u]
[name_f]Angelina[/name_f] or [name_f]Angelica[/name_f]
[name_f]Naomi[/name_f] or [name_f]Noemi[/name_f]
[name_f]Ilaria[/name_f] or [name_u]Hillary[/name_u]
[name_f]Mabel[/name_f]
[name_f]Audrey[/name_f]
[name_f]Winifred[/name_f]
[name_f]Felicia[/name_f] or [name_f]Felicity[/name_f]
[name_f]Dorothy[/name_f]
[name_f]Lucia[/name_f] or [name_f]Lucinda[/name_f] or [name_f]Lucille[/name_f]
[name_f]Matilda[/name_f]
[name_f]Idalia[/name_f]
[name_f]Amara[/name_f] or [name_f]Amanda[/name_f] or [name_f]Amadea[/name_f] or [name_f]Aimee[/name_f] or [name_f]Amabel[/name_f]
[name_f]Juliet[/name_f]
[name_f]Clara[/name_f] or [name_f]Clarisse[/name_f] or [name_f]Clarinda[/name_f]
[name_f]Phoebe[/name_f]
[name_m]Alexander[/name_m]
[name_m]Benedict[/name_m]
[name_u]Merritt[/name_u]
[name_u]Bodhi[/name_u]
[name_u]Jesse[/name_u]
[name_m]Matthew[/name_m] or [name_m]Mathias[/name_m] or [name_m]Matteo[/name_m]
[name_m]Helio[/name_m]
[name_u]Arlo[/name_u]
[name_m]Nathaniel[/name_m]
[name_m]Lysander[/name_m]
[name_u]Galen[/name_u]
[name_m]Wilfred[/name_m]
[name_m]Aaron[/name_m]
[name_u]Haven[/name_u]
[name_m]Clement[/name_m]
[name_m]Lucian[/name_m] or [name_m]Lucien[/name_m] or [name_m]Lucio[/name_m]
[name_u]Beau[/name_u]
[name_m]Raymond[/name_m]
[name_m]Theodore[/name_m]
[name_m]Felix[/name_m]
[name_u]Valentine[/name_u]
[name_m]Isaac[/name_m]
[name_m]Archibald[/name_m]
[name_m]Jonathan[/name_m]
[name_m]Solomon[/name_m]
[name_m]Aurelio[/name_m]
[name_u]David[/name_u]
[name_m]Frederick[/name_m]
[name_m]Amado[/name_m] or [name_m]Amadeus[/name_m] or [name_m]Amadeo[/name_m] or [name_u]Amory[/name_u] or [name_m]Amias[/name_m]
Some options work perfectly for both genders 
Good luck!
