Complex sentence: The Felbrigg estate was left to Windham's wife Cecilia Windham in the first instance for the remainder of her life, were after William Windham's heir was Vice Admiral William Lukin.
Split into simpler sentences: The Felbrigg estate was left to Windham's wife Cecilia Windham in the first instance for the remainder of her life. Thereafter, William Windham's heir was Vice Admiral William Lukin.