Commentary for Yevamot 10:9

אם לא ידע הראשונה משהגדיל ראשונה חולצת ולא מתיבמת – for the levirate relationship of two levirs is upon her, for since he did not know her [sexually] once he grew up, she did not leave from the first falling [of the levir to her].

If a boy of the age of nine years and one day had intercourse with his sister-in-law, and after he had come of age he married another woman and then died, if he had not known the first woman after he had become of age, the first one must have halitzah but may not be taken in yibbum, while the second may either have halitzah or yibbum.
Rabbi Shimon says: [the yavam] may perform yibbum with whichever one he wants, and he must perform halitzah for the other [woman].
[The same law applies] whether he is of the age of nine years and one day, or whether he is of the age of twenty years but had not produced two pubic hairs.
The final mishnah of chapter ten contains another dispute between the sages and Rabbi Shimon over the status of an act of intercourse performed by a nine year old boy. As we have learned before, according to the sages, such an act partially acquires the yevamah as his wife. According to Rabbi Shimon, it is doubtful whether the act acquires her as a wife, but if it does, she is fully his wife.
Section one: When the nine year old boy has relations with his yevamah, according to the sages, he partially acquires her as his wife. If he later dies, without having intercourse with her from the time he reached majority age, she may not have yibbum, as we learned in the previous mishnah, since she is tied to two yevamim (the minor and the brother who first married her). Had he had intercourse with her after having reached majority age, he would have completely severed her ties to her first husband, and she would be able to subsequently have yibbum.
Concerning the woman he married after having reached majority age, she may have either yibbum or halitzah. The question we must ask about this clause is why we would have thought that she is not allowed to have yibbum. The issue is connected to that which we learned in 3:9, concerning an adult who performed ma’amar with his yevamah, but before he had a chance to have yibbum with her he died. There we learned that just as she must have halitzah and not yibbum (since she is tied to two yevamim), so too must her rival wife. Our mishnah deals with a similar situation, where a man dies having ties equivalent to ma’amar with one wife, and full marriage with another wife. Either our mishnah disagrees with the opinion held in 3:9, or it makes a distinction between the two situations, holding that in one case the woman may have yibbum and in the other she may not.
Rabbi Shimon holds that an act of intercourse performed by a minor doubtfully acquires the yevamah as a wife. If it does acquire her as a wife, then she is fully his wife and she may have yibbum. If it does not acquire her as a wife, and he has not had relations with her since, then she was never the minor’s wife, and the woman may have yibbum with his brother (in this case she is still tied to the first brother). In either case, the yavam may have yibbum with either woman. He must have halitzah with the woman with whom he does not have yibbum or halitzah, lest the intercourse of a minor does not acquire. If this is so, then the second wife is not the rival wife of the first wife. If he had yibbum with the first wife, the second wife still needs to be released. If he had yibbum with the second wife, the first wife still needs halitzah on account of her previous marriage to the first brother. Whichever wife did not already have yibbum lest the intercourse of the minor does acquire, and then the surviving brother would have had yibbum with two yevamoth, which is forbidden.
Section two: The final section of the mishnah relates to the entire second half of the chapter (mishnayoth 6-9). A boy who reaches nine years of age is considered a minor until he reaches puberty, or until he reaches twenty years of age. According to the talmud, this is actually true until he reaches the age of 35. If at 35 he still has not shown signs of puberty, he is classified as a natural-born eunuch.

רבי שמעון אומר מיבם לאיזה שירצה – for Rabbi Shimon does not hold by the levirate relationship of two levirs , and these words, we mentioned in the chapter of “Four Brothers” (Tractate Yevamot, Chapter 3, Mishnah 9).

וחולץ לשניה – for they are not rival wives to exempt one of them with the levirate marriage of her colleague. But a levirate marriage of both of them does not occur since she is her rival wife as part of the statement of levirate marriage of the Rabbis, but it appears as two widows of a brother who died without issue who come from one house.

אחד בן תשע ואחד בן כ' – both are equal for everything mentioned above, for as long as they haven’t brought forth two [pubic] hairs, he is a minor. Until he will be thirty five years of age and if he arrive at the thirty five years and did not bring forth two [pubic] hairs, even though they didn’t appear on him, they are a sign of his being castrated/a eunuch that is explained above (see Tractate Yevamot, Chapter Eight, Mishnah Four) – which is that of a natural eunuch.