Halakhah sur Job 36:8

One who has a brother of the same father, even if he is a bastard or an apostate, even if he is a child, [the law is that] from the point that his head and most of his body already was born before his brother died, behold he is tied (Rama: [Zokek] meaning "tied", like (Job 36:8)) to his [brother's] wife in levirate marriage. Even if he is on his deathbed, or diseased so much so that he will not live because of it, she cannot remarry [to someone else] so long as he lives. But if it is a brother of the same mother, or he was born after [the married childless brother] died, his "yibbum" can remarry [lit. "permitted to the marketplace"]. Similarly, if [the married childless brother] has a brother from a maid-servant, or a gentile woman, he is not tied to his [brother's] wife in levirate marriage, even though he was born in holiness, since his conception was not in holiness.