Emmadubois's list "If There Were a Sister" of 4 great name ideas: Sola - Ysabel!

If There Were a Sister

Baby sister to William (Liam, Leelee, Leo) and Colette (Coco - Cokie - Letty - Co - Coconut)
: CB approved

Sola

Tamsin is an offbeat name occasionally heard in Britain and just waiting to be discovered here. U.K. actress Tamsin Greig is a star of the show Episodes, Tamsin Olivier is the daughter of Joan Plowright and Sir Laurence Olivier..
 Tamsyn is the alternative medieval Cornish spelling. Other spellings are represented by actresses Tamzin Outhwaite and Tamzin Merchant, and by writer Tamasin Day-Lewis, sister of Daniel.
 Tamsin is a 1999 fantasy novel by Peter Beagle.

Tamsin

Theona is a feminine name with Greek origins, generally considered a feminine variation of Theon, which means 'godly' or 'divine.' The name carries elegant ancient connections while maintaining a modern, melodic quality. Historically less common than similar Greek-derived names like Theodora or Theodosia, Theona has experienced modest increases in usage in recent decades among parents seeking names with classical roots but distinctive sounds. The name's connection to divinity gives it an ethereal, spiritual quality. With its flowing sound and relatively uncommon status, Theona offers a balance between uniqueness and accessibility. The name works well across multiple cultures and has particular appeal for those drawn to names with meaningful Greek etymology and a gentle, feminine sound.

Theona

Ysabel is the original spelling of this ever-more-popular name, though to English-speakers, the initial Y may confuse pronunciation. Only 14 baby girls were given the Ysabel spelling in the US last year, while Isabel and Isabelle were each given to about 1800 babies.