My Highland Warrior (Warriors of the Highlands 1) by Miriam Minger

Fearsome Scots laird Gabriel MacLachlan must take a bride, but not just any bride. Magdalene, his liege lord's younger sister is known to be as bonny as any Highland lass but as mad as a loon.

Mad Maggie, they call her-and Gabriel has no choice. Not if he wants to put food in his clansmen's bellies, clothes on their backs, and repair the crumbling castle he calls home. A fat dowry in exchange for a madwoman, whose life will be in danger from the MacLachlan family curse the instant she becomes his wife.