Love is a powerful kind of magic, but “Practical Magic 2” asks a more interesting question. What happens when that love isn’t romantic? The sequel to the 1998 classic puts Sally and Gillian Owens back at the center of a story that explores love in all its forms. While romance still plays a role, the heart of the movie belongs to the sisters and the complicated ways they protect, challenge and sacrifice for one another. At its best, “Practical …