The May bride : a novel of Tudor England / Suzannah Dunn.

Enhanced descriptions from Syndetics:

Jane Seymour, a shy, dutiful fifteen-year-old when her eldest brother, Edward, brings his bride home to Wolf Hall. Katherine Filliol is the perfect match for Edward, as well as being a breath of fresh air for the Seymour family, and Jane is captivated by the older girl. Over the course of a long, hot country summer, the two become close friends and allies, while Edward is busy building his own allies at court and advancing his career.Only two years later, however, the family is torn apart by a dreadful allegation--that Katherine has had an affair with Seymour patriarch--made by Edward against his wife. The repercussions for all the Seymours are incalculable, not least for Katherine herself. When Jane is sent away, to serve Katharine of Aragon, she is forced to witness another wife being put aside, with terrible consequences. Changed forever by what happened to Katherine Filliol, Jane comes to understand that, in a world where power is held entirely by men, there is a way in which she can still hold true to herself.