Not really, Sand in the pokemon games is usually associated with the Ground type. Trapinch and Hippodown as examples, as well as Sand Attack and Sand Tomb.Would be nice, but Castform doesn't change with Sandstorm, and if it does, wouldn't it just also just as well become Rock/Fairy, or Steel/Fairy?
And if you say that it should get a Form with Sandstorm, then it should get a Form for Fog too, IMO.
But since Sandstorm isn't really Weather, I don't see it happening.
I've no idea why the Sandstorm attack itself is Rock-type though.
Rock/Fairy could potentially work, but Steel/Fairy would make no sense, since Sandstorms have nothing to do with Steel. (Steel types just resist it because they are made of metal and their skin isn't damaged by it)
Sandstorm is really weather, both in real life and in Pokemon games. Fog is rather uncommon and is basically just an environmental effect more than a weather condition, then again, I wouldn't mind if it WAS a Castform form, Flying/Fairy maybe? i can dream.