The Eye of Midnight by Andrew Brumbach*

Cousins, William and Maxine, meet the first time at their grandfather's house (whom they do not remember).  Battersea Manor is especially formidable since their grandfather is not there to greet them.  Together they explore the mansion, discovering some unusual and creepy artifacts.
Once their grandfather appears, they are whisked away to New York City where evil men lurk, awaiting the chance to terminate their grandfather's life.  The cousins must band together with a new friend, Nura from Turkey, to thwart assassins from infiltrating the city.
Very exciting.  The characters are quite admirable and likable (especially William!).
Based on a real group of assassins-the Hashashin who placed terror in the hearts of the Middle Easterns thousands of years ago.
-Borrowed from the library.
*Highly recommend for junior high and older (see the warning).
Warning: Quite violent at times.