Tractate Yevamot - Chapter 11 - Mishnah 4 יבמות פרק י״א משנה ד׳

If the child of a woman was mixed up with the child of her daughter-in-law and the mixed-up children grew up and married women and then died, the [other] sons of the daughter-in-law perform halitzah but may not contract yibbum, for it is uncertain whether she is the wife of his brother or the wife of his father’s brother. The [other] sons of the older woman either perform halitzah or yibbum, since the only doubt is whether she is the wife of his brother or the wife of his brother’s son. If the not-mixed-up sons died then [with respect to the widows of the sons of the older woman] the mixed-up sons perform halitzah and may not have yibbum, since it is uncertain whether she is the wife of his brother or the wife of his father’s brother; [With respect to the widows] of the sons of the daughter-in-law one performs halitzah and the other [may] have yibbum.