Simple. She doesn't. She never has, and she wants to kill him now. Anyway, it was confirmed that they are siblings and Morgana was the first to know (apart from Uther xD)
During the first few seasons, she did 사랑 him, but it was no 더 많이 than a sibling 사랑 which later turned to hate as fear of persecution turned Morgana against him and everyone else in Camelot