In order for the commandment of yibum to come into effect, the two most basic components are for a married man to die with no offspring, and to have a living brother who can marry his widow, thus fulfilling the mitzva. The Mishna on our daf teaches that having any brother will allow the commandment to take effect, except for a brother that was born to a non-Jewish woman or slave; similarly, having any child will keep the commandment from taking effect, except for a child that was born from a non-Jewish woman or slave.
The Gemara interprets the use of the expression “any brother” and “any child” to include even the case of a brother or child who was a mamzer – a child born as a result of an adulterous or incestuous relationship – who ordinarily is not allowed to marry into the Jewish community (see 23:3) – deriving this from the passage u-ven ein lo ( 25:5) that the mitzva of yibum takes effect when he has no son. The expression ein lo is interpreted by the Gemara to mean ayyen alav – investigate his situation carefully; that any evidence of offspring will eliminate the mitzva of yibum. This interpretation is explained by the Sma to mean that the Torah requires us to check that truly ben ein lo – there is no evidence whatsoever of a child.
Although the Gemara clearly indicates that the teaching of the Mishna comes to show that a mamzer is considered a legitimate sibling or child with regard to the rules of yibum, the geonim and rishonim add another case that needs to be considered. What should the halakha be with regard to a sibling who has become an apostate? Do the rules of yibum still apply? Will we insist that the widow refrain from marrying anyone else if the apostate refuses to participate in a Jewish religious ritual? Similarly, if the apostate brother dies with no children, will his wife become a yevama to his brothers (assuming, of course, that he was married to a Jewish woman)?
The first approach to these questions was to affirm that Jews remain Jews, even if they committed sins as severe as apostasy. Later on there were some geonim who suggested that we must distinguish between a situation where the brother’s apostasy took place before or after the wedding. In the event that already at the time of marriage the brother was an apostate, the suggestion is that the marriage took place conditionally with an understanding that the apostate brother was not to be included in the family for these purposes.