The Harry Potter book series follows a young wizard as he begins to train and harness his powers and skills. Although Harry Potter doesn’t have any siblings JK Rowling revealed in an interview that Lily, Harry’s mother, was pregnant with a second child when she was killed by the dark lord. It has even been said that
Lily had convinced James to name Severus as the godfather to their second child. If things had gone differently Harry very well would probably have had a little sister but there is no indication that he ever would have had a brother. It is still up in the air as to whether this can be considered theory or fact.