Trying to remember the title of a book I read 10-15 years ago as a teenager. It's about a girl who buys a horse (I think it's a piebald horse) from a man who has either a son or a nephew living with him. The son/nephew's name is Grey or Gray and he and the girl work with the horse together (training it for jumpers) and end up falling in love. The girl lives with her sister and there is a scene in the book where her sister's horse is chasing her horse around the paddock.
I've been wracking my brain for hours trying to remember the title!!