@Borelius
I’m pretty sure Alice’s mother was the one who put the curse on the Duke. My guess is it was for one of two reasons:
1) To ensure that Alice would be the only female around the Duke and increase the chances of them getting married. In that case, my guess is that the spell will be broken when he touches Alice. It would be proof of his true love for her.
2) To ensure that the Duke could never get into a relationship with Alice. Basically, she wanted to protect Alice from the Duke for some reason that hasn’t been revealed.