The Girl Who Saw - Not Alone

What if Harry had family... sort of? What if he had a sibling-type person that, until he attended Hogwarts, he didn't know about? What if she hadn't been allowed to contact him until he learned about the wizarding world? And how will she approach him once they meet? Meet Marina Callisto Black. (I suck at summaries! Longer one in introduction!) (M-Rating to be safe...)