What is the book? It's a variation of Tam Lin?

A variation of Tam Lin and the lords daughter is going to a ball and tam makes her a dress that is very beautiful, but only because it makes others see in her what he see's . At the ball the kings... Brother? Falls in love with her because of the magic and comes back to the castle with her. And she sleeps with tam and gets pregnant. When it is noticed she's pregnant she is tried for adultery by her fianc�. Then she says he's been sleeping with her maid so he's the same. And in the end she faces the fairy queen on Halloween to Tam back.