The Secret of his Heritage
After the accident, Harry Potter's twin was forgotten. The authorities assumed the death eaters got him, and decided that Harry would not be told to prevent the reactivation of their link. However, Dumbledore decided that Harry should be told if the twin ever returned to Hogwarts. After Dumbledore died, McGonagle found his name on the list and immediately alerted the ministry. After all, his name would not be there if he were dead like they assumed. The ministry was already infiltrated by then, and they immediately set out to find Harry's long lost twin. The ministry set out to find Percy Jackson.
I do not own either of these books. They belong to Jk Rowling and Rick Riordan. This is simply the result of insanity and Wattpad.
P.S.: This will not be exactly according to the plotline. I was only able to read the Deadly Hallows once because I had to borrow it from a friend. If I get it wrong, please feel free to correct me in the comment section. I am going to try to check it out.