Rabbi, Moshe (Rabbeinu) wrote to us that if an ACH of someone should die and leave behind an isha UVEN EIN LO (and there is no son to him) YEVAMAH YAVO ALEIHA ULEKAKHAH LO LISHA VHAYAH HABECHOR ASHER TELED YAKUM AL SHEM ACHIV HAMET (the brother of her husband must come to her and he must take her to him as wife and the Bechor she bears shall raise up the name of the dead brother).