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.
Even a relative, or woman, or slave, or child who can understand, they are believed to say, "So-and-so is the brother of so-and-so from his father's side, and that is his levirate wife," and they do halitza or levirate marriage by their word.
Converts or freed slaves have no brothers at all, even if one of them was conceived in an unholy manner but the birth was holy [i.e. of completely Jewish stock], and the other was conceived and born in holiness. And even if they are twins and they were [both] born in holiness, they are not brothers unless both were conceived and born in holiness.
[a woman who requires Yibum] proposal plan is with [the deceased's] brother-in-law who converted [to another religion from Judaism], there are some authorities who permit if he was converted when his brother was married; she does not have to rely on him [meaning he is free from any form of obligations to levirate marriage i.e. yibum,chalitza] REMA: who is he? if she sins and marries without challitza, since its not know to her that there was a brother-in-law, and afterwards its known that she has a brother-in-law who converted, she is not free [from the available options in regard to the obligation of levirate marriage]. there are authorities who say if there is an option to do Chalitza to her [she should do it] to end [the halachic relationship between the two of them]. if there were two brother-in-laws, one converted and is a Gadol and one is a Katan and is a Yesroel, No chalitza [from the converted brother-in-law] rather from the Yesroel; and even post facto [if she did chalitza with the converted brother-in-law] he is not survived with the converted brother-in-law. But If he was converted by force, he [the deceased] is survived by him [halachically] its permitted post facto [to do chalitza/yibum with him]. and the one who marries a woman and he has a brother who converted, he is able to marry and stipulate with a condition twice that if [he dies and] the proposal plan is before the [brother] who converted to do levirate marriage- they will not be married.
if the diseased was an individual who converted from Judaism, his wife is in need of the brother [to preform levirate marriage/chalitza]. THIS IS THE RULE TO BRANCH [from his brother] THAT THERE IS [this obligation] TO HIM FOR A JEWISH WOMAN
Information of the deceased's brother was not withheld, and he [the deceased] said:"I have a brother" he is not believed. some one comes and says "I am the deceased's brother" he is not believed; Even if one witness of a relative comes to testify that this individual is the brother of the diseased, they are not believed and she is free from any form of levirite marriage.
If someone was known to have brothers and at the time of his death he said: I have no brothers, he is not believed. So too if he said about someone who was known to be his brother: This is not my brother, he is not believed.
If he was not known to have brothers and a rumor started that there are witnesses who will testify that he has brothers and the witnesses are in another country, even if he said at the time of his death: "I have no brother." She [the widow] must still be concerned and should wait until the witnesses who said [he has a brother] come and are asked.
A woman whose husband has neither a son or a brother, and her mother-in-law and father-in-law are overseas when her husband dies is permitted to remarry; for we are not concerned that her mother-in-law may have given birth to a son and he is entitled to her [via levirate marriage]. But, if her mother-in-law left when pregnant, we are concerned lest she gave birth to a boy and he is entitled [to her via levirate marriage].
A woman whose husband dies, and he had a son overseas, she is permitted to [marry] a non-family member [i.e. not her husband's brother. We are not concerned that the son may have died, rather she stands in her place by presumption [of the son being alive]. The brother of a deceased husband may not live with [his brother's widow with whom he will do] halitza if he is attracted to her, just like an engaged [but not married] couple. (Responsum of the Rashba #209); this is specifically after the halitza ceremony, but, prior, it is permitted (Maraharm Padua paragraph 7).