Chestnut Hill: #3 Heart of Gold by Lauren Brooke
Welcome to Chestnut Hill, a boarding school where every girl's favourite subject is riding. Honey's made close friends at school yet she hasn't told anyone that she has a twin, or that he's really ill. Then when Patience's new pony is injured, no one understands her dedication to a pon that's not even hers. But Honey doesn't know how to share her secret...