It's about as grammatical as "Where is your mother going to?". It's perfectly fine to have a preposition as well as the interrogative, because where, as demonstrated, doesn't just ask for an adverbial, but also for the object of a prepositional phrase determining a location. (The core idea of where).
As for is redundancy, I can't immediately think of situations where it necessarily clearly states the difference of movement vs static (I.e. to vs at), but that doesn't mean you can't say it. The fact that it's in common practice in some dialects makes it just as usable. However, if you don't want to be associated with those dialects/want to speak "Standard Oxford English", I suggest you don't use it. It isn't in common practice there.