Elizabeth's older half sister, mary (later known as Bloody Mary) was executed on november 17th 1558 due to the fact that she was not the ideal leader. Later, Elizabeth's mother had passed due to illness leaving her to receive the throne. Suprisingly, Elizabeth was not the one who was suposed to get the throne. She was actually last in line. When those before her had passed, she was left to receive the crown.
Bloody Mary (Mary), was executed from being punishable of the crime treason. Her son accepted her beheading, making the execution of Queen Mary final. Because she seeked to return England to it's Catholic origins, she percicuted hundreds of Protestants and earned the nickname Bloody Mary for her ways.
Elizabeth I received the throne after her half-sisters passing when she was 25. She was still royalty when she was younger, but didn't receive the throne through marrige. The only way that she could receive it would be if the people before her died, and which they did, and according to the family tree, she was the last in line to receive the crown.