Does Sylvanas even know that Calia is still "alive," for want of a better term? Knowledge of her recent resurrection seems pretty rare - Jaina is pretty high-placed in the Alliance leadership and even she's only heard rumors (and those may only be about Calia's actions during the "Before the Storm" and not her later raising into undeath). I think Sylvanas's more direct concern would be Kul Tiras itself, and the Proudmoore Admiralty leadership specifically.