Siana is pretty - See-ah-nah (I think this is the Anglo-Welsh pronunciation; I don’t like the “aw” sound of that Americans o sometimes use for an “Ah” sound, it sounds harsh/ saw-like to me. If not See-ah-nah, I would think it was See-anna or Shah-nah).
I like the sound Alessa but in Australian-English, it evokes, “a lesser” (as in less-than), so I would use Alessia instead. Or I’d use Alessandra and nickname to Alessa. (There’s Alyssa too but I don’t warm to this name.)
Some other names similar to both:
Sienna
Sian (Sha-n)
Samara
Soraya
Aurelia
Tessa
Elara
Alora
Elyra
Aria
Good luck! 