A family is torn apart by fierce belief and private longing in this unprecedented journey deep inside the most insular Hasidic sect, the Satmar.       Sweeping from the Central European countryside just before World War II to Paris to contemporary Williamsburg, Brooklyn, I Am Forbidden brings to life four generations of one Satmar family.     In 1939 Transylvania, five-year-old Josef witnesses the murder of his family by the Romanian Iron Guard. He is rescued by a Christian maid who raises him as her own son. Five years later, Josef rescues a young girl, Mila, after her parents are killed while running toward the Rebbe they hoped would save them. Josef helps Mila reach Zalman Stern, a leader in the Satmar community, in whose home Mila is raised as a sister to Zalman's daughter, Atara.