Yevamos 4:11-12

Yevamos 4:11

Let’s say that four brothers are married to four women and all the husbands die. (In this scenario, there are at least two more brothers surviving those who died.) If the eldest surviving brother wants to marry all four of them through yibum, he may do so. If a man with two wives dies, performing yibum or chalitzah with one exempts the other. If one was fit to marry a kohein and the other one was not and the brother wishes to perform chalitzah, he should do so with the one who is not able to marry a kohein (because performing chalitzah with the other woman will remove that option from her). If he wishes to perform yibum, he may do so with either woman.

Yevamos 4:12

If a man remarries a woman he divorced (and she had married someone else in the interim), or if he marries a woman with whom he performed chalitzah, or if he marries the relative of a woman with whom he performed chalitzah, he must divorce her and the child of such a union is a mamzer (product of an incestuous or adulterous relationship); this is the opinion of Rabbi Akiva. The Sages say that the offspring of such a union is not a mamzer. They agree that if a person married the immediate relative of a woman he divorced, the child is a mamzer.