Complex sentence: Lord Shannon's heir and only child Grace became by marriage The Countess of Middlesex (d.1763) and by Will and by virtue of her husband dying childless in 1769, the estate passed to her cousin Colonel John Stephenson and thereafter his sisters.
Split into simpler sentences: Lord Shannon's heir and only child Grace became by marriage The Countess of Middlesex (d.1763). By her Will and by virtue of her husband dying like her childless in 1769, the estate passed to her cousin Colonel John Stephenson and thereafter his sisters.