Simplify the following text for a Grade 12 native English-speaking person.
Original: When the 15-year-old King lay dying in June 1553 , he nominated Jane as successor to the Crown in his will , thus subverting the claims of his half-sisters Mary and Elizabeth under the Third Succession Act .
Simplified: When the 15-year-old King lay dying in June 1553 , he nominated Jane as successor to the Crown in his will , thus subverting the claims of his half-sisters Mary and Elizabeth .