Simplify the following text for a Grade 5 native English-speaking person.
Original: Lady Mary Grey ( 1545 â `` 20 April 1578 ) , sometimes spelled Marie , was the third and youngest daughter of Henry Grey , 1st Duke of Suffolk and Lady Frances Brandon .
Simplified: Lady Mary Grey ( 1545 - 20 April 1578 ) was the sister of Lady Jane Grey and Lady Katherine Grey . Her parents were Lady Frances Brandon and Henry Grey .