Sian is not pronounced the same as
Sean! At least, I've never heard it that way over here.
Sian is with a long ah sound like the word Shah with an "n" on the end.
Sian is fairly common in Britain (especially Wales) and there are some well known people like the weather presenter
Sian Lloyd and the BBC newsreader
Sian Williams.
I agree it's pretty. And I also dislike the name
Shawn.