You have to love Andre Norton and the Witch World. This is like the bazillionth Witch World novel. You don't have to read them in order-- though I keep thinking it would be fun to try. Aisling, a powerful young witch, is forced to defend Kars by overthrowing her older brother, Kirion, who is using his power to ensnare the Duke. She finds romance (just a smidge-- Andre wrote science fiction/fantasy in the 50's when she took a man's name just to get published-- so no explicit sex or anything like that)and unexpected allies as she goes.
Good addition to the Witch World series
Aisling is a young witch who fights to protect her homeland of Kars. Unfortunately, the biggest threat to Kars is her older brother, Kirion, who has chosen to use his powers for evil, and years ago forced Aisling into exile. With her younger brother Keelan, helping her, Aisling returns in deguise, to undermine Kirion's power and defeat the evil Duke of Kars.