I don't know what "Look who she is" means in English!
'Look who she is' is the order of words for a statement or imperative in English. 'Look, who is she?' would be the order of words for the question.
Since this sentence is affirmative, but it includes an indirect question, I was wondering whether ydy should be considered as a consequence of the relevant direct question (i.e. an interrogative ydy ):
Pwy ydy hi? → Edrychwch pwy ydy hi.
or as a consequence of the identification construction that uses ydy / yw :
Xxxx ydy hi. → Edrychwch pwy ydy hi.
In the latter case, could ydy be replaced by yw ?
Xxxx yw hi. → Edrychwch pwy yw hi.
If you heard a knock at the door and sent your roommate to "Look who it is," how would that differ from this sentence (or is this context applicable here?)
If a girl auditioned for a play, was assigned a character, and the character list was put up on a noticeboard, then one friend could tell another to "Look at who she is!" But that expression is very informal. // I'm not always understanding when to write "look" and when to write "look at".