Yevamot, Chapter Thirteen, Mishnah Eleven
This mishnah continues the subject discussed in the previous two mishnayoth.
1) [If one was] of age and the other a minor, and the yavam had intercourse with the one who was of age, and then he had intercourse with the minor, or a brother of his had intercourse with the minor, he does not disqualify the elder for him.
2) If the yavam had intercourse with the minor, and then he also had intercourse with the one who was of age, or a brother of his had intercourse with the one who was of age, he disqualifies the minor [for him].
3) Rabbi Elazar says: the minor is to be instructed to make a declaration of refusal.
Sections one: In this case one of the widows was of majority age and one was a minor. If the yavam has intercourse (yibbum) with the one who is of age she is fully acquired to him as a wife. If he or his brother subsequently have intercourse with the other widow the act is forbidden (for she was his brothers wife) but does not effect the status of the one who already had yibbum.
Section two: If the yavam first had intercourse with the minor, there is a doubt whether or not she is acquired to him. If he then has intercourse with the one who is of majority age, that act would be an act of yibbum if the act of intercourse with the minor did not acquire her. Since the older widow may be his wife through yibbum (or his brothers wife) he is forbidden to remain married to the minor widow. He also cannot be with the older widow, lest the minor was successfully acquired through yibbum.
Section three: Rabbi Elazar states that in such a case we should instruct the younger widow to make a declaration of refusal, thereby annulling the marriage. This would allow yibbum with the widow of majority age.