nanaea's right--Sian is pronounced "shan." Pronouncing it si-ann would be like naming a child
Juanita and pronouncing it "JAW nit-a."
In the top 1000, there is a name, Cyan (like the color) that pops up. It's also in other spellings like Cyanne, Sianne, Cyann etc. Why not go with one of these variations?