Yevamot 100 - Questionable paternity
If a woman did not wait for three months after her separation from a husband, and married another man and gave birth - then the paternity of the child is in question.
If, in addition, with both men she had more sons, then it is not clear if the child is the brother of the first family or of the second one. Therefore, if he marries and dies childless, there is a question of who should do yibum with his widow. The answer is that both sets of brothers have to give her a chalitzah, because he may have been their full brother, and she is therefore part of the family.
However, nobody can do a yibum with her. That is because he may have been only from the same mother, but not from their father, in which case his widow is forbidden as a former wife of their brother, while the mitzvah of yibum exists does not apply, because it is only for paternal brothers.
Art: Dressing The Baby by Jacob Simon Hendrik Kever