Getting Past Discrimination

Harry goes back to Hogwarts for his eighth year, to have visions. The true meaning of these visions: He has a twin sister. And the worst part, she's in Slytherin and is dating Malfoy. Another surprise: Marlene McKinnon, a dead Order member is actually alive... What are the challenges that Harry and his sister will have to get past to get past this? And at what costs?