Troubles with Mary's Accession to the Throne
Mary was the only one that Edward desperately wanted to remove from the line. His main worry was the fact that she was deeply faithful to Catholicism. He knew that were she to take the throne, she would try to reinstate papal authority over England and reverse the reforms made by Henry VIII and himself. However, he had to remove Elizabeth as well even though she was a Protestant because the reasons that he used to justify Mary’s removal also applied to Elizabeth. Therefore, his decree would not hold up if Elizabeth was not included because it would obviously biased and easy to poke holes through. In order to have some sort of legal validity to the removal of his sisters from the line of succession, Edward signed a Letters Patent on June 21st 1553.
In this document it stated that Elizabeth and Mary are not fit to rule because they are illegitimate. Henry’s marriages to both Catherine of Aragon and Anne Boleyn were not legal and therefore, Mary and Elizabeth cannot inherit the throne. He also iterates that women should not be able to rule because they are subject to their husbands’ rule. He emphasizes that this would be even more dangerous if they were to “marry with any stranger born out of this realm” who “would rather adhere and practice to have the laws and customs of his or their own country or countries be practiced or put to use within this realm.” This will remain a concern of the entire country when it sees Mary and Elizabeth on the throne. He placed the sons of Lord Francis and their male heirs as the first in line and Jane Grey and her male heirs as the second in line.
It is interesting to note that Edward sees no contradiction in reasoning that his sisters cannot rule because they are women yet he still institutes a woman as a successor. This seems to go against what was written his Letters Patent. However, since Jane Grey was to be married to Guilford Dudley, son of John Dudley who was the head of the Privy Council, it did not matter as much. The power would be in the hands of a family that Edward trusted and not in the hands of a foreign ruler. John Dudley clearly had a scheme to cement his and his family’s authority in England.
Technically Edward was not supposed to interfere with the line of succession as stated in Henry VIII’s Act of Succession. It was treason to do so. Edward changed it anyway, but the fact that Mary had been instituted as successors by her father and an actual Act of Parliament helped her garner support when Edward VI died in 1553 at the age of 15.
 Robert Tittler and Susan L. Battley. The Local Community and the Crown in 1553; the Accession of Mary Tudor Revisited. (Historical Research 57, no. 136 (1984): 131-139.)
 Edward VI, Letters Patent (1553)