Jude Deveraux the Conquest

Lovely Zared Peregrine was the pride of her family, a treasure her rough-hewn brothers would strive at any cost to protect from their ancient enemies, the Howards. The Peregrines had suffered loss enough. Thus Zared's brothers trained her in the arts of war, and dressed her in boy's clothing. Beyond the castle walls, none knew that the youngest Peregrine was a girl....

Yet when the magnificent Knight Tearle Howard returned from a lengthy sojourn in France, it took him but one glance to discover that Zared was most definitely female, and beautiful. Now, as the enmity between their families raged on, Tearle would mount a bold campaign to win the ravishing spitfire's heart -- and save her from his brother's dark and deadly obsession!