Not technically... But it's easier to say yes than to discuss Sylvia's true nature.
I don't think Sylvia was ever on the ship. That's Isis.
(Whose true nature is less known than Sylvia's, so I suspect OP confused the names instead of the characters)
Damn. Cat who is actually a woman identification failure and Classic Star Trek episode identification failure all in one tiny post.
Happens to us all eventually... :)
(via @neil-gaiman) :)