Yevamot 39 - Whose right is most in her?
One cannot marry his brother's wife, even if they ever divorce. However, if that brother dies, now every brother can marry her, in a procedure called "yibum," which stands for having relations with your late brother's wife.
The right (and the obligation) to do so rests primarily with the firstborn brother. However, it is his choice whether to marry her or to release her by performing a "chalitzah" (she unties his shoe and spits in front of him). Let us say he announces that he wants to release her through "chalitzah".
Meanwhile, a younger brother suggests that he will do a yibum. Normally, sleeping with a woman with the intention of marriage completes the bond. Here, however, the younger brother is a minor (not thirteen yet), so he would acquire her through the power of her previusly being married to the late brother. So now the question is,
whose suggestion is the better one?
On the one hand, the intention of the firstborn brother should be decisive, because he has the right of first refusal. On the other hand, he sends her away, whereas the younger brother wants to keep her in the family, which is the primary way to fulfill the commandment.
The argument is not applicable today. Initially, they did the yibum not for pleasure, procreation, or money, but for the sake of the mitzvah. When people became more mundane, the law was changed and releasing her with "chalitzah" is now the only authorized way.
Art: A young man teasing a sleeping woman by Ottilie Hornig Von Geyer