Something that's never mentioned, since Ginty's gift is made clear as being for her, is that the horse needs exercise and a certain amount of expertise. I suspect the whole "it's Ginty's" only lasts as long as the holidays and Rowan gets to use it later on when everyone is back at school - if nothing else, Mrs Marlow could produce that as a suitable excuse for keeping it exercised. And the horse is of a size that means that women could ride it, but the twins might have a certain amount of difficulty.