A magic item that allows the gypsy to change shape…hmm.
Maybe a medallion? Something that can be worn around the neck, and then when in horse shape, attached the the horse’s bridle. Or if a different shape, something small attached to a collar (if the gypsy turns in to a cat or a dog). Or an earring of some sort. (Just googled “dog with earring” in images – the first three images. Yikes.) Or maybe it’s not metal. Maybe its something woven, which could be tied in the animal’s hair or around its neck. Horses are easy to tie stuff to; dogs/cats, not so much.
Then there’s the ritual. I suspect it’d have to be short and easy, for I want my hero to be able to turn back into a horse in a pinch and not rely on the sister to do the ritual. But the item is key. Maybe the initial shape shifting requires an elaborate ritual, but each time after, there’s just a short reminder ritual.