Under My Spell by Deborah Wright
Cara is twenty-five years old and beautiful, intelligent and kind. She's also a witch. Unsure of her feelings towards magic and desperate to escape the clutches of her overbearing mother, she takes a 'proper' job as a nanny for the Wilkins family. But things don't go quite as she has planned. Cara moves into the Wilkins' home and before long the whole family come to rely on her as a source of fun and excitement in their lives. Nick and Andie Wilkins' marriage is falling apart and so Cara sets about trying to reconcile them. When her benign attempts to fix things result in Nick flirting with her and Andie contemplating an affair, Cara can't resist any longer. She resorts to the magic she's been trying so hard to to give up and soon the whole house is in uproar...
Good secondhand condition, paperback.